I also think Ewan McGregor would make an excellent Lupin! I've always pictured a younger Jonathan Pryce as Sirius Black- too bad he's probably too old to play the part now...although, with the heavy toll Azkaban has taken on him, perhaps his aged appearance might be appropriate after all...
The rumours about Tom Felton not being in PoA are not true- just another internet rumour.
Richard Attenborough is lobbying heavily for the role of Dumbledore; IMO he'd be a good choice. Ian McKellen would be an even better choice, but I somehow doubt he'll do it. I find it hard to believe...