jenn - as far as your example goes, if R+L=J ends up holding true (and the consensus among fans seems to be that it will be confirmed at some point, probably if/when we ever meet Howland Reed), i do hope they put that scene in. in fact i'd be disappointed if they took out/changed any of Ned's flashback from the Tower of Joy, but that's also because that whole exchange between Ned and the three kingsguard is one of my favorites in the series
this is looking ahead of book 1, but i could see them maybe leaving out some of Arya's misadventures with the BwB, just because there are a LOT of very minor characters introduced that will just bog down the show.
and to the above poster, yes there's been a hard date given, July 12th. i'm assuming that GRRM is close enough to finally being finished that the publishers are giving this date as a deadline to him as much as it is a release date for us. but that's just a hunch