BG and BG2 have been available on GOG.com for awhile, so what'll this contribute? According to the developer, original game content. Overhaul Games told Joystiq : "This is the Enhanced Edition, it's not a repackage. We have the original source code and are doing a significant update." I've separately reached out to the developer for more information.
Right now, the game's announcement page is still busted, so I'm not sure whether to tell you to keep F5ing or not. I'll update with any additional information as we receive it.
Update: Overhaul has passed along the raw text from the game's website, which I'm pasting here. Trent Oster also revealed in a tweet that the Enhanced Edition will include the Tales of the Sword Coast Expansion. "Since its initial release in 1998, Baldur's Gate has entertained millions of fans around the globe, and has received countless awards. This classic saga of mystery, intrigue, and adventure has set the standard for Dungeons & Dragons® computer roleplaying games ever since. Overhaul Games has assembled a talented team of artists, programmers and designers to enhance this timeless classic. To remain true to the spirit of the game, the team includes original Baldur's Gate developers. Baldur's Gate: Enhanced Edition™ and Baldur's Gate II: Enhanced Edition™ will feature a re-forged version of the Infinity Engine with a variety of modern improvements."
Below, someone captured this image from the announcement page before the website's server collapsed.