I'm pretty sure most (if not all) flash memory cards have built-in wear levelling these days - it's a feature built into the card itself so the controller/OS doesn't have to concern itself with this function, although some filesystems (such as jffs2 which is the filesystem used for the flash memory built into the 770/N800) are optimised to minimise updates where possible.

There is a discussion in bugzilla where a Nokia claims the flash memory in the device should be good for 30+ years assuming it is used every day. With a filesystem that is not optimised for flash it might be possible to assume that the lifetime of the card will be less than 30 years, but probably longer than a couple of years (which would be fine with me). I'll try and find a link...

EDIT: Bug #598 discusses wear levelling.

EDIT2: This wiki entry claims that SD and CF cards have built-in wear-levelling and error correction circuitry... no mention of MMC though.