My MacBookPro's a late '07 model, but still runs Portal quick enough. Only downside is the portals themselves are 'black' and don't show what's on the other side of them. Ah well, any excuse to upgrade...! It works a treat on my iMac, well, I started playing it a few months back via Bootcamp.
Oh and I TOTALLY agree with Spiceware - if you haven't played it, don't google ahead for spoilers! :)