The following is a news story taken from a 1up article that describes some information that GamesRadar previously posted up. The information was taken down shortly after it was posted up.
According to the preview, main character Chris Redfield will be accompanied throughout his ordeal by an as-yet unnamed female mercenary (probably one of the two new female character recently revealed in Famitsu). This character will be guided by the AI when going solo, but a second player can take control at any time. This includes online co-op play, with seamless "jump in/jump out" design. (We're assuming this game will actually have voice chat, unlike the woefully miscalculated Resident Evil Outbreak games.)
And speaking of mercenaries, the preview also revealed that the shoot-em-up Mercenaries mini-game from Resident Evil 4 will be making a return. Gameplay-wise, the preview reports that while the game will still lack strafing, it will add two contemporary features: a cover system to press up to walls and shoot around corners, and the ability to duck and dodge using the right analog stick.
The preview also revealed some new story details as well (and you might want to consider these some really small spoilers, just in case you want to know as little as possible going in). The game will apparently take place in a nameless African country near a line of active volcanoes that stretch across Eritrea, Ethiopia, and Kenya. When the game begins, everyone will at first be uninfected, and this time around it'll be a long-dormant virus released through volcanic activity that turns the locals into zombie-like killers.
Moreover (and you might consider this an even bigger spoiler), the preview states the game will feature a number of classic Resident Evil characters (with Albert Wesker and Sherry Birkin being confirmed), including flashback episodes that'll fill the player in on character histories that haven't been explored yet.
One last bit of information, perhaps just as noteworthy as the co-op, is buried at the end of the preview: that the game will be released in 2009. Many have speculated whether RE5 would make it out this year, but is this casually mentioned date confirmation that it won't see release until next year? Let's hope not...
Also, it is mentioned that the online Co-Op is supposed to be for the entire game.
UPDATE: Full GamesRadar article text with new pics is posted at THIA.
Also the Official Resident Evil site now has a 72 hour countdown.
UPDATE (5/28): 3 new hi-res screenshots on the Capcom Blog.