The TV producers have ruled out flashbacks. Apparently they considered them, but rejected them for being too confusing and adding too many characters to a show already overloaded with them.
In fact, they did film one flashback scene for Season 1 (in which we see the Mad King Aerys torturing Brandon Stark to death), but it was cut for this reason, as well as time.
My thought is that they will use Bran's weirwood abilities to get around this limitation, and show some 'flashbacks' as visions instead.
GRRM has expressly ruled out a Robert's Rebellion prequel series. The new deal with HBO has been done to allow them to develop (with other writers with GRRM in a producing capacity only, as on GoT) a DUNK AND EGG project (whether a TV movie or ongoing series is not known at the moment). It also sounds like GRRM will be proposing a TV series based on his TUF VOYAGING space opera series to HBO as well.I read something very interesting the other day on the westeros forum. GRRM is signed on to do additional work on top of the already existing GOT series. This has to make me wonder since we have been told GOT won't be giving flashbacks in the series are they going to do a new (mini) series? Possibly on Robert's Rebellion. I would expect that to be a major hit to both book loyalists and people who have no idea about the Rebellion. I guess the Dunk and Egg books are a possibility for an adaption but i don't see that translating to the screen quite as well. Anyways should be interesting whatever the case.