Chiming in with Mayday and Haz. I have the MacBook Pro 15 for travel and iMac for home. Great combination.
I think with the iPad you'll miss not having the physical keyboard (although you can add one) and a camera. Great portability but lacking as a "full-time" laptop. If you are dead set on going the ipad route, I'd wait for the the 2nd Gen one coming out Mar/Apr; it'll have a camera(s) plus other performance improvements. I'd recommend the 3G+wireless version.
The new MacBook Airs are very popular and give you the best of both worlds if you are worried about weight. Get an Airport Express and the lack of ethernet port won't matter; you can turn any ethernet cable (hotel w/o wireless) into a wireless network in your room. You won't need a superdrive (you can get an external one); you can move files and music thru file sharing on the network. You'll pay a premium for the solid state HD. Personally, I like more display real estate for things like BidX (my eyes aren't as good anymore) so 13 would be my min size. Airs have no illuminated keyboard; a must for me.
For me the 15 Pro is the best of all worlds for my needs. The new Airs are pretty awesome but it came down to screen and keyboard for me. Take a look here at some comparisons and points to ponder:
New MacBook Airs