Three causes why I don't believe in this:
1. Chewbacca has died. I don't remeber the books anme but it is in the official SW timeline. and this timeline is already full of events until his death. It would be very difficult to find a gap for a three episodes sequel except they film the Zahn-books.
2. Everybody could play Chewie. You even can make him a CGI (see Monsters Inc.). That he (Mayhew) now appears in Episode III is a bonus for the old fans. If there would be sequels not to many people would care who is the man behind the mask.
3. If there really will be a TV-series about Darth Vader, I can imagine that chewie (Mayhew) will be part of it. Maybe this for is the backdoor arrangement in the contract.
But there are some more rumours. One is that the contract between some publisher of toys etc. has been lengthened til 2012. The question: Why so long if there are no more movies. Films sell toys.
Additionally Mark Hamill has stated that he can imagine to play a part in at least one of the sequels. any hint ? Or just something an ex-actor said who would like to be back on the big screen.
In contrast to some other people here I would like to have sequels. Someone said: Sequels suck. But TESB didn't suck. Aliens didn't suck. There is still a chance episode 7,8,9 wouldn't suck. Especially if there is an other director. (Steven Spielberg's name was mentioned a few times). To be honest: I have no fear the prequels could suck more than the sequels.
So let's lean back and wait for the things to come. Let's hope for the OTs on DVD, Special Editions of the Prequels that are better than the originals and for the prequels. "Nothing is written." (Peter O'Toole, Lawrence of Arabia)