All the talk of cost per Mbyte is rubbish. Ignore cost when doing your memory format trade study until the very end. For your average 5Mpix camera on high res mode (no compresion) a 256MB memory module will hold at least 75 pictures. Medium resolution doubles or triples the picture capacity. Thus for your average point and shooter...a single memory card is plenty for your typical picnic outing. I personaly find it is better to mangage a large picture taking session, or a long vacation on multiple memory cards. This way I can ensure a failure of one doesn't wipe out all and it lets me organize by time or subject.
MMC/SD is the media format of choice hands down due to small size, low power consumption, popularity and, yes, reasonable (falling) cost. As with all memory cards write and read speed is a consideration but if speed is needed there are variations in all formats that are optimized for speed that cost about 25% more than the standard speed devices. Cost per megabyte is small. Consider all your other devices that you own now or will own. PDA, cell phone, digital video recorder, and others. I have standardized on MMC/SD and now have about 10 various capacity cards all interchangable. A 256MB SD card costs only about $60 these days.
If you have a choice, stick with the best format out there: MMC/SD for all your removable memory needs.
Final note: A consortium of camera manufactures have attempted push consumers towards yet another memory format call xD. This format is synonomus with Sony's proprietary Memory Stick (and all its variations). While a nice technology, one buying into this format will need other formats for all their other memory hungry toys as few to no non-camera companies are using this format.
If you have a choice, stick with MMC/SD for all your memory card needs. As for digital cameras, consider the Pentax Optio series...owesome.