It would be best if they had more than just a walk-on role.
The first one I remember seeing was The Process starring the late Kimo Leopoldo.
Here is the unique plot breakdown.
"When martial arts champion Pinoy travels to America, his fighting skills are put to the test when he finds himself in the middle of a violent gang war. Forced into sudden street combat against fierce members of the Crazy Dragons gang, Pinoy saves the life of Jesse. Impressed with Pinoy's amazing fighting abilities and grateful for his life, Jesse befriends the stranger, introducing him to his father. But this new found friendship is put to the test when the Crazy Dragons surround Jesse's home, threatening the lives of his family. Suddenly the two must protect what they cherish, and with the help of Kimo summon all their martial arts strength for an action packed ultimate fight to the death."
*List movies that should never, ever be seen by human eyes.
Those crappy tapout movies where Heath Herring is world champ and he fights uber Mir in a parking lot 
15 minutes Oleg Taktarov
Predators again Oleg
Rich Franklin was in Cyborg Soldier, Genesis Code, and he plays Coach Pruitt in Hamil(according to IMDB).
There are plenty of crappy movies out there with MMA guys. The Russian Bear has this shit on lock down tho, he's starred in movies with Dinero (twice), Pacino, Harrison Ford, Joaquin Phoenix, Duvall, plenty of others.
Catch Forrest's latest movie "13" where he's in it for all of a minute, and they actually dubbed over his voice, made me laugh.
Jardine has had a few cameos that Gamer movie with Gerard Butler comes to mind.
Oleg and Frye are the only two that really come to mind as far as main stream movies tho.
Old Russell Crowe/Denzel Virtuosity movie (Sid in the bad ass purple suit) pretty sure had Shamrock maybe another few guys in there. Was a future cagefighting type event.
No Rules had F.Shamrock & Couture, I really dont recommend it. RedBelt. Universal Soldier (new one) had Arlovski.
what was that politically unpopular war movie that Tito starred in? After which he spent years of his life on a P.R. campaign "fighting for the troops?"
Circle of Pain is the one where Herring fights Mir in a parking lot.
Herring is EXTREMLY uncomfortable acting around women. His sex scene was distrubing to watch.
Tito and Kimo in "The Dog Problem" w/the guy from the Boiler Room and James Caan's son...funny flick!
NEVER SURRENDER HAD SEVERAL. INCLUDING A DUBBED VOICE FOR GSP.
ranier wolfcastle - <b> NEVER SURRENDER</b> HAD SEVERAL. INCLUDING A DUBBED VOICE FOR GSP.
Oh, man. I skipped this gem. Any clips of the dubbed voice part?
Never Surrender was like a skinomax flick. It was funny though.
RKing85 - Circle of Pain is the one where Herring fights Mir in a parking lot.
Herring is EXTREMLY uncomfortable acting around women. His sex scene was distrubing to watch.
But he is NOT a homosexual