As per an interview with Masachika Kawata & Jun Takeuchi at SDCC 09. From The-Horror.com
No Wesker Will Not be back.
Their comments on the matter.
Resident Evil producer Masachika Kawata said in a panel at the San Diego Comic-Con that popular series villain Albert Wesker is not coming back. When asked if there was any way Wesker could survive the events at the end of Resident Evil 5, Kawata firmly stated, no.
"Even if you want Albert Wesker to come back," Kawata told the crowd, "there's no chance he's coming back from that."
This was then confirmed as the "definitive answer" to the possible return of Wesker.
UPDATE AUGUST 2009
The answer is unknown. Resident Evil is switching producers after Darkside Chronicles, and apparently they are going to need to reboot the series all together. What that means is likely you won't see any of the past cast in the upcoming game, but you may see Wesker along with some of the commonly used good guys back in future Resident Evil's.
Either way Kawata won't have a vote of say, and rumors around are speculating that the team does want Wesker to return.