Partly this depends on the file size created by your camera, of course. For my 1Ds, I use two 2-gig CFs and a 1-gig CF for my main shooting. They usually get me through a day, though on shoots with lots and lots of wildlife behavior opportunities I'll go through more, and draw on a hodgepodge of old 512 meg cards I used to use with my D60. Each evening, images are backed up to a laptop hard drive, and duplicated on a USB 2.0 external laptop drive. The laptop stays in my car trunk each day (assuming I'm traveling) and the USB drive stays stashed in my hotel room.
The risk of putting everything on one card doesn't only arise from card failure. Twice in the field I have dimwittedly formatted a card with shots on it. (I know, I'm a schmuck!) Having multiple cards is a lot better--Worst-case I've only lost part of my shots for the day, and even if (as has happened both times) I am able to recover the lost shots with rescue software, I can put the formatted card aside and continue shooting with other cards. With one big card, I'd be shut down till I could go in with image rescue software.
In a crisis, I'll use battery power or an inverter to run the laptop and download cards to the laptop out in the field, but this doesn't happen often and I don't like to do it. It was confusion during this process that caused one of the formatting incidents I talked about above. At a really productive place like Bosque, I go back to the hotel for lunch in mid-day, and do a leisurely download of cards there.