From what I've read 'round the net mac's pretty much take care of themselves and as far as space issues go, external drives are pretty cheap now.
As long as you know how to keep your browser cleaned down of all the chaff it picks up on the net, and providing your system is running o.k, I wouldn't have thought that any kind of maintenance program is really necessary.
Running Windows Vista used to require a weekly maintenance schedule to keep it running sweet (as sweet as it got, anyway)
I've had the macbook 4 months now and maintenance seems pretty non-existent and it runs fine (is that tempting fate?)