Now with the advent of Apple's Front Row media center application, album artwork is even more important because it's prominently displayed on your iMac G5 while you're listening to iTunes.
Although you get artwork with tunes purchased from the iTunes Music Store you don't get album artwork when you rip your CDs to iTunes because Gracenote's CDDB database doesn't provide it.
That said there's a cottage industry developing for tools that allow you to populate your album artwork to better take advantage of your color iPod. A search on MacUpdate.com for "album artwork" yields several applications from free to commercial that will handle the task but some are better than others.
My favorite tool is MPFreaker ($20) from Lairware because it's accuracy is better than the rest. I noticed while jamming to my "four star 80's" playlist while unpacking the other day that a lot of my artwork was less than perfect due to experimenting will several inferior tools. My artwork was a mess! I don't think that a Reba McEntire album belonged with Edie Brickell's "What I Am," for example. Several of the errors are actually quite funny. In addition to album artwork MPFreaker will automatically find and add album name, genre, year and track number too.
If you want to take it one step further you can also clean up all your anonymous mislabeled MP3 with names like "Track 01" with an MP3 fingerprinting tool like Script Software's idTunes which compares a few bytes of the song with their online database of over 3.5 million songs and populates your ID3 tags appropriately.