There aren't a lot of MMOs that fall into the "I've got a few minutes and want to goof around in" category. Second Life to me is one. Sure, we can argue the "Is SL an MMO argument" while we're taking a break from the "Which is better: Mac or PC?" argument. For me, SL is good for that hour I've got to kill because I got to my night class early, or I'm sitting in bed with my laptop and my RSS feeds are up to date. Really, some days I'll play anything to avoid a daily quest.
The gotcha is, when I got my laptop I had a hard time convincing my wife err myself that I needed to blow almost two grand on a laptop. I still don't have too much buyer's regret over my regular Macbook. Games aside, the three apps that do most of my heavy lifting are Twitteriffic, Mail, and Scrivener (these days, it seems like I write more than game). World of Warcraft runs OK on it, but, lordy, does Second Life crawl on it. I'm not sure which gets better FPS: Second Life running on a Macbook, or frozen molasses rolling uphill. Yeah, I know, the integrated video on the 'book isn't supported and that's what I get for running unsupported hardware and I shoud be using a Macbook Pro. Duly noted.
That said, an unscientific study conducted at my school's cafeteria and library, taken randomly on Mondays between 4 and 5 pm shows a heck of a lot more college kids sprung for the cheapo 'books. Second Life could be considered a good game for non-gamers in the way The Sims was considered a good game for people that hated Quake. So, lets take a good, hard look at what's involved with Second Life on Macbooks, what you can do, and what realities your going to face running SL on a non-Pro 'book.