How you backup your iPhoto library depends a lot upon how large it is. Mobile Me might work for a small library, but it could be too slow for a large one. Mine is nearly 31 GB and I have another 18GB of JPEGs that have not been imported into iPhoto, so the total would be about 50GB. You can easily drag and drop the iPhoto folder (or the entire Pictures folder) to an external hard drive. Even if you update iPhoto, you can still get the individual image files from that folder.
Once you have backed up the iPhoto folder, you can use Time Machine to keep any changes or additions and/or you can use a file/folder sync application like the aptly-titled File Synchronization to keep the backup current.
Here’s what I do: I use a card reader (not the camera—way too slow) to download the image files onto a 1TB OWC external FW400/800/USB2/eSATA HD. (I usually shoot JPEG + RAW and put the RAW files in a separate folder.) I go through the images (often with Image Browser from Canon or Graphic Converter’s Slide Show option) and pick the ones I want to import into iPhoto. Those get imported, so I automatically have two copies of any photo I thought worth keeping. Periodically, I archive all new image files on the external HD to DVDs, which gives me 3 copies of the ones in iPhoto, 2 copies of the others. So far, there’s been no need to clean up the OWC HD. It has over 118 GB of photos, going back about 2 years. It’s much larger than the iPhoto library because it has the RAW files (about 6.6 MB each) as well as the JPEGs that are in iPhoto.
A while back, I bought a LaCie Little DIsk Design by Sam Hecht 500GB FW/USB2 portable HD. It’s bus-powered* and quite compact (5” x 3-3/8” x 11/16”)—and not very expensive ($130-140). I synch the iPhoto LIbrary, iTunes Library, Documents folder, the photos folder from the OWC HD, and some others onto it. That way, I can get access to all my photos, music, etc from my 13” PowerBook G4, which has only a 60GB HD.
* It works very nicely on FW 400. Like many USB drives, it may need to use two USB ports for power, especially on a laptop. That’s one reason that I always connect it via FW—plus, FW400 is faster than USB 2.