On Symbian phones (have done this on an N95 and an N97) there is a bunch of files you can download to your PC from somewhere and then stick in the relevant folder in the phone memory, using the USB cable on "mass memory" setting.

Alternatively you use the "Nokia Maps Loader" application on your PC. There is a Mac version too but I found this didn't work (surprise surprise).

Exactly how you go about it on the N900 I don't know but I think there's a thread on here somewhere - do a search for "maps".