I'm in the same boat as you... I will be getting a Digi 003 i two weeks and I just got a macbook pro. I've never used a mac before but I think it's silly to say that Tiger won't work on it. Sounds like a music store guy trying to sound knowledgeable, to me.
I was thinking about calling Apple and asking if they'd send me Tiger for free...

I will say...

I work with a guy who is a top level studio engineer and he is totally in Pro Tools. He says he's heard that compatibility with Leopard is imminent. Some say we won't see it til summer so you can't trust anything.

This post isn't much help, but you most certainly can install Tiger on your MBP... (Someone step in if I'm wrong) Also know that you have to backup your drive because all your data will be erased if you install Tiger.