The cost of flash memory has plummeted to what must be a new low, or something close to it, of $13.13 per gigabyte. (Not every camera can handle such a large card; check your manual.)
That's less than a tenth of what I paid for my first gigabyte card—used! I suppose it's only because of convention and habit that cameras don't come with built-in memory, like an iPod. We started off with separate cards, so we continue with separate cards, even though they may not strictly be needed any more. Maybe in the future a card drive will be considered an optional feature in cameras, only available on some models and not others.
Mike (Thanks to Michael Tapes)