Gamespot seems to have found the bottom line to this whole Sonic mess . Check this out:
http://gamespot.com/gamespot/stories/news/0,10870,2873412,00.html

Sonic situation clarified

The original Sonic Adventure will indeed be appearing on the GameCube.

A few days ago, it was initially reported that the original Sonic Adventure was being brought to the GameCube by Sonic Team. Sega of America subsequently released a statement explaining that it would be an entirely new Sonic the Hedgehog game and not the original Sonic Adventure as previously thought. However, it appears that Sega of America made a mistake, and that the new Sonic game for the GameCube is indeed Sonic Adventure.

We'll have more as it becomes available.

http://gamespot.com/gamespot/stories/news/0,10870,2872973,00.html

New Sonic games bound for GameCube, GBA

The next installment in the Sonic the Hedgehog franchise and more are coming Nintendo's platforms.

Sega has announced that it will release the next installment in its Sonic the Hedgehog franchise for the GameCube. According to a representative from Sega of America, the game will be a brand-new addition to the franchise instead of a remake of the original Sonic Adventure for the Dreamcast, as was first suspected. In addition, Sega is also developing Sonic Mega Collection for the GameCube and Sonic Advance 2 for the Game Boy Advance.

"Building upon the excellent worldwide sales and incredible success of Sonic Adventure 2 Battle and Sonic Advance, Sega is determined to develop and release new titles in the Sonic the Hedgehog series that will appeal to the growing audience of gamers worldwide," said Tetsu Kayama, chief operating officer, Sega Corporation.

While no details on the games have been disclosed, Sonic Advance and Sonic Mega Collection are expected for a release within this year in Japan. Both games will also be showcased at the upcoming World Hobby Fair held later this month in Japan.

Sonic Team also announced that Sonic Adventure 2 Battle for the GameCube and Sonic Advance for the Game Boy Advance have shipped 1 million copies each worldwide.

http://www.gaming-age.com/cgi-bin/news/news.pl?y=2002&m=6&nid=30-81.db

Nintendo seems to be the company of choice for Sonic Teams blue hedgehog hero; Sega has announced titles spinning the critter onto the hardware makers platforms. Sonic Adventure the original) and Sonic Mega Collection are headed to the GameCube and Sonic Advance 2 for the Game Boy Advance.

Japan will be receiving Sonic Mega Collection and Sonic Advance 2 at the end of the year, while Sonic Adventure does not yet have a release date. The contents of the proposed collection or what types of upgrades we can expect from the Sonic Adventure port have not yet been announced, nor has any prospective US releases

From the August issue of "Electronic Gaming Monthly", there is a rumor page that states that Sonic is not having any more "Adventures". It will go back to its original roots, which means that there will be no more treasure hunts for Knuckles. A release date remains sketchy at this point, Sonic Team may show us a few pics of the new game by the end of the summer. For more info, check out the August issue of EGM. The cover has either Mario, Link, or Samus on it.

Well not exactly. The new August Issue of Electronic Gaming Monthly stated this:

"Sega's blue, attitude-filled, sneaker wearin' mascot has gone on his last Adventure.
But dry those eyes and buck up, lil' camper. He's not gone for good. Sonic Team is reportedly hard at work in Japan on a brand-new Sonic that will take the 'hog back to his roots'. That's right-no more freakin' searching for jewels as Knuckles. The gameplay will return to the tone of the original series, focusing on fast platforming action with no B.S. And to reflect the change in design, the game will reportedly drop the Adventures moniker it has clung to for the past few years. While a release date remains sketchy at this point, we could see see the first fresh fruits of Sonic Team's labor by the end of the summer..."

So what I mean is that. Sonic games will no longer be an adventure. It will go back to it's roots and maybe become the best 3D Sonic game of all time. Lets hope for the best.

Sonic Team announces plans to reveal the next game in the Sonic series at the upcoming World Hobby Fair.

Sega has announced that Sonic Team will be revealing the next game in the Sonic the Hedgehog series at the upcoming World Hobby Fair in Japan, which will be held on July 13 and 14. Unfortunately, there are currently no details regarding specific platforms. Sega also announced that Super Monkey Ball 2 will be playable at its booth and that it will also be offering a rare chao download for showgoers.

We'll have more as it becomes available.

Keep in mind that these are just rumors I've heard floating around, so only time will tell us if they're true or not.

The new gameplay will be closer to Sonic3&Knuckles then it has with the Sonic Adventure series.
On a similar note, all the levels are universal, so every character plays through all the levels.
Sonic, Tails and Knuckles are playable. Amy, Eggman and Rouge appear, but if they're playable or not remains to be seen.
--Shadow might be alive. I hope not, for the sake of decent storyline...

That's about it. I got it from an oekaki board, but the news was taken down about two weeks ago, so wherever they got the news from is a mystery.

Sonic Adventure 3 [GC, XBOX]:

Thanks to an "Industry Insider" for going behind closed doors to bring us back info on Sonic Adventure 3 for Xbox and GameCube.

Naka-san told viewers of this top secret video that the all new Sonic Adventure will put Knuckles, Tails, Amy, Rouge, and Robotnik specific stages to rest. Instead, all zones will be tailor made for Sonic and Shadow, although both Tails and Knuckles will both be playable to run around those same levels in different ways.

Finally Naka-San described his new camera system as "the best camera system ever seen in any platformer ever". We eagerly await more details.

Perhaps it's Sonic Adventure they way most of us wanted it? All action!

Remember - this is NOT CONFIRMED yet by Sega or Sonic Team - so don't take it as fact yet.