The below sources claim Robert Pattinson is to have a cameo on Entourage, September 12th 2010.
**UPDATE: According to GossipCop’s report this is now believed to be false.**
According to TV@Spreadit.org:
Entourage season 7 episode 10 called “Lose Yourself” which was directed by David Nutter and written by Doug Ellin did not air this evening on HBO but will be broadcast on September 12, 2010.
As you eagerly wait for Entourage’s heated finale which will explain how the friends end up losing all they have worked for – here is the preview of the upcoming installment:
Mrs. Ari is still fuming over Ari’s angry meltdown the night before, Ari tries desperately to keep her surprise party on track. The guys attempt to stage an intervention for Vince, who has gone off the deep end. Turtle scrambles to find investors to save the tequila business. Eric receives an unexpected request from Terrance McQuewick, which leads him to consider Scott’s business proposal. Robert Pattinson guest stars as Ari’s most difficult client ever.
From eWorldPost.com:
There are more reasons for a person to tune into HBO on the 12th September. Apart for the usual crew of the show that has amused the people over a long time, a surprise is expected. It is surprising but true that in this episode, the star of the Twilight Saga, Robert Pattinson will make a guest appearance.
Thinking of Rob 
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When I saw the pictures of Rob at a party with Bob Saget and John Stamos from Full House and everybody was wondering why he was hanging out with them, I commented on another site that he was possibly going to guest star on Entourage because both of them have been on. How exciting, hope it is true!
oh i see the whole story now, thanks shirlee!
it’s gonna be great if i could see new character of him!
the most difficult client..ummmm…how exciting!!
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Oh do I hope this is true. he and Jeremey Pivin . . . this could be the best thing since the MTV awards commercial. All digits crossed!
is it true???