If you were hoping you could save some cash on that $169 Rock Band package by buying only what you need or want, we've got some bad news. The accessories won't be sold separately until early 2008. Until then, even if you have two guitars from Guitar Hero, you'll have to pony up for the full guitar + drums + mic package, meaning you'll have an extra axe that you'll have to eBay off. [TheBBPS]
IIRC, the GH controllers should work with Rock Band, but there are some features, like the 5-way effects switch and the second set of fret buttons that you won't have — but those don't really effect gameplay.
I'm more interested in using the Rock Band guitars for Guitar Hero.