OK.. so I know this rumor has been circulating forever. However, thanks for Supernatural Hunters' Addict for posting this, IMBD has listed Jensen Ackles as Leon Kennedy in Resident Evil 4 (rumor). Check this out, click here.
IMBD is usually a good database source. But I can't find anything that will confirm this. And certainly nothing from the studio Screen Gems (Sony). BeyondHollywood.com has some articles in relation to the script development of the movie, but nothing about casting or whether the movie is a go.
I guess we will find out when it becomes true. Let me know if you come across anything on this.
If you don't know who Leon Kennedy is or what Resident Evil is, check it out here.