I'm going to buy the new RB3 drums, but I'm not sure what the best approach to pro mode is.  I could play some stuff (not perfectly or anything) on hard with the normal drum set in previous rock bands.  Should I keep playing hard drums and get up to expert before doing pro mode on songs, or should I just jump into pro mode right away and leave the difficulty at easy or medium?

I was thinking about this myself.  I've been hearing that switching to Pro you will be 1 skill level lower.  So if you're Expert on normal Drums, you'll probably be on Hard for Pro Drums.
 
When I get my cymbal pack, I'm probably just going to switch Pro right away and go down to Medium from Hard.  I want to get used to the cymbals right away.  Plus it will be more fun!

If I were you, I'd just make the jump right away. It's really not a whole lot different. Your biggest challenge will be developing memory of exactly where the cymbals are, and you'll probably swing and hit air a few times as you're getting used to it. I've actually never used the regular madcatz cymbals but that was the toughest part for me when I got my IONs.

Just go for it straight away.  I played RB2 with the cymbals, and it really only took me a couple of weeks to get comfortable with them.

Rock Band 3 drums? I thought there was only the RB 2 drums with the cymbals you buy separately and then the ION drums.

" Rock Band 3 drums? I thought there was only the RB 2 drums with the cymbals you buy separately and then the ION drums. "

Just jump right into pro-drums, as you'll want to get used to hitting cymbals as soon as you can.  Moving from expert RB2 drums to expert RB3 pro drums still has me messing up when I'm supposed to be hitting cymbals but I instinctively go for the pads.