Most of the stuff in the FRCS is OK as it is, and clearly doesn't need to be updated for 3.5. Actually, I'd be glad to see a new version of the FRCS, but only because of the severe errors in the original print (check the errata to see what I mean).
All the updates will center around the crunchy bits (i.e. prestige classes, feats and spells, mostly), so I'm thinking they'll probably conjure up the crunchiest FR book every, giving a bit more spells, feats, PrCs (possibly, even fixing a couple of those given in Magic of Faerun and Silver Marches) and giving background information on the realms more suited for players than DMs. That way, players will buy it for the crunch (and useful data for FR campaigns), and DMs will buy it for the updates.
All of this, of course, is just conjectures based on my tush talking too much.