Hey,
I don't know about Portal 2, but I have Portal installed on my late 2008 13" Unibody Aluminium Macbook (2.4GHZ/4GB RAM/9400M) It plays great actually. No lag (maybe the first time you shoot a portal or stare into one for a while). It's defiantly playable

Full screen mode there is a frame rate drop (so I play in 4:3 window). The only time the game slows down for me is if I increase the field of view. I can't remember my exact settings but the graphics look really good

. The fans do go crazy & the laptop gets hot but just put some music on, and put something between the laptop and your legs
I'm waiting for the PSN to go back up before I can download the free steam copy of Portal 2 for my laptop. I'm not a serious gamer but I think that Portal 2 runs on the same engine (i think) as Portal, which was released in October 2007. So I think it should be fine
I'd have uploaded some pictures but I can't figure out how to take screenshots without the menu coming up.
Settings are:
Portal render depth: 2 (recommended)
Aspect Ratio: 4:3
Resolution: 800x600
Model Detail: High
Texture Detail: Very High.
Shadow Detail: High.
Water Detail: Reflect World.
Shadow Detail: High.
Color Correction: Disabled.
Antialiasing: 8x MSAA
Filtering Mode: Anisotropic 16x
Wait for Vertical Sync: Enabled.
Motion Blur: Enabled.
Field of View: 75.00 (lowest setting).