I'm using normal Windows 7 laptop at the moment but tossing up ideas on what Mac I should buy for this task in the future.
Lots of reputable designers are switching their setup to MacBook Air + iPad.
One of them went from MacBook Pro to iMac 27" down to a top spec 13" MacBook Air. Depends really what you're doing.. if you're not working with PSDs over 700MB on a regular basis and your storage system (he uses external 1TB passport in addition to 256GB Flash on the Air) is good then you'll be fine.
He backs his files up regularly with a RAID and two in the cloud. Pretty smart and it's a really lean setup.