Hey John,
If you are at all handy/artistic check out an arts supply store, they will have sheet products that will 'stick' to the front plastic of the iMac. It comes in sheets, almost like the same way plastic wrap works...
You should try to locate the arts district or studios in your area... you can have something painted onto the film and the film can be applied to the face of the iMac. There will be a couple of trouble areas, first is the trimming but that can be overcome, second is the heat generated right above the Apple logo on front bottom of iMac. I don't know if the heat will prevent the film from sticking or the film somehow insulates and won't allow the machine to dissipate heat from that area.
Another option is to actually have the front face painted.
If you need help locating an artist/colony let me know, I pretty much have some knowledge of someone somewhere...
Rick
iMac G5 iSight 20" - 30G iPOD in Slimming Black - Mac OS X (10.4.7) - HP Pav 15" WS and Toshiba Sat 17" WS LP's - Canon 20D & A620