You've played it on the Wii?

http://www.rockband.com/compatibility

There's my proof.  Thanks for playing.

The compatibility breaks down like this:

Guitar Hero World Tour Guitar works with Rock Band 2 and Beatles Rock Band.  This has a slot for the Wiimote to go into.

Rock Band guitar comes with a USB adapter.  This only works with Rock Band games.

Rock Band drum sets only work with Rock Band games.  Guitar Hero World Tour drum sets work with everything, but it's a little strange playing Rock Band with it because you use one of the cymbals as a tom, and it's just not natural.  Plus the Rock Band games are 500% better than the Guitar Hero games, so just go with RB stuff and you'll be fine.

There's no point in buying an ION set if you're only going to play casually.  Unless you're worried about getting 100% on every song, it's pretty pointless.  The one that comes with the games is fine.  It will miss a note every now and then, but if you're just playing to have a good time, it's not a deal breaker.

I personally am going to get the George Harrison guitar as my second guitar just because of how it looks.  I've used the Rock Band guitars and the Guitar Hero World Tour guitars and personally I don't really have a favorite.  As long as they're full size, they work fine.  The smaller guitars get a little annoying (especially the Guitar Hero 2 White guitar.  The bodyshape is weird and it digs into your side.)

Okay, I'm done with this long rant.  Hopefully you have my new number if you have any questions.