I habe been discussed the best way to keep his data safe and accessible anytime anywhere you are with my friend Fred (fredbrunel.com) last week.
He exposed that we required to have some kind of duplication to an external drive, but this caused him some trouble.
My way to handle this is pretty simple.
For years now (I mean since 1990 basically), I am always keeping all my data in one location, in one HD. That is, my laptop internal drive.
As I am equipped with a MacBook on Leopard, I use time machine to make "realtime" duplication on an external drive that stays at home 100% of the time, while i carry my macbook all the time in my Crumpler backpack. The external is currently a USB2 1Tb WD Book drive which I bought for a mere 170€ if I remember.
The only point here, is to cope with the ever growing disk space required by the data over years.
My solution is quite straight: every 6 months or so, I simply buy the biggest internal drive on the market and replace the one I currently have. This way I increase of about 50% the disk space every 6 months. THe cost of a disk is typically always the same 130€. And you have to replace it by your own, which is quite easy operation that takes 10mins to say the truth.
To make the transition really seamless, I just make a time machine backup, shutdown my macbook, replace the HD and boot from the external drive using the last time machine backup. It takes a while to restore the HD, but you can have a beer in the meantime (or anything you think's best suited to you).
Once this is completed, your setup is ready for duty. And I keep the former internal HD in a safe place offsite as a rescue backup.