Michael Hollick, the voice actor behind Grand Theft Auto IV's Niko Bellic, will reprise his role in the upcoming Grand Theft Auto V.
This tidbit of news comes from IMDB, which said that Hollick was currently filming his role in GTAV, but the post was recently taken down. Niko was the immigrant protagonist of GTA IV who came to Liberty City in search of revenge in the open sandbox game. He will probably not be the main character in GTA V, since the narrator of the teaser trailer didn't have an Eastern European accent, unlike Niko. See for yourself:
The trailer did have a character who looked like Nico(the homeless man holding a sign), but that is more likely a homage to past games rather than any sort of hint. No clue how big a roll Niko will play in the game; he could be a huge part of the story or a passing cameo. GTA V will take place in Los Santos, Rockstar's version of Los Angeles, and probably sometime after the events of GTA IV, if Niko's inclusion is any indication.
Another past GTA protagonist, Carl C.J. from Grand Theft Auto IV: San Andres, is also supposed to play some role in the upcoming GTA. His inclusion is a little easier to understand, since San Andreas was the first game to include Los Santos. We may even see Claude from GTA III, but his character was mute during the game, so no voice actor will be returning in that role.
We'll have to see how these characters will play out, the game is rumored to come out in 2012.