I was going to write 'cheap trick' but didn't want to add 'and I don't mean the band.'
Anyway, because I am all for improvising and spending as little money as possible:
Grab a canister of those glass cleaning wipes (stainless steel wipes will also work). You really just need a single wipe to wipe down the keyboard/laptop body.
All that black grime? Comes right off. Really amazing. Should be most notieceable on the white Macbooks (obivously), but the black Macbooks also pick up grease and grime, so it wouldn't hurt to try it on those.
You do not need to scrub hard at all, either. Just turn off your laptop, wipe it down, let it dry for an hour.