There are only three models of car that can use the carpool lane, and two of those models are hybrid-only models (Prius and Insight). Even the Civic has unique wheels and an obvious rectangular badge on the back that you can see for quite some distance. The chances of someone with a non-hybrid Civic trying to steal the stickers (which are designed to peel apart is you attempt to remove them), steal the wheels, and steal the hybrid badge from a Civic hybrid are pretty slim.
All the stickers that will be issued in CA have already been issued, period. There won't be any more, and they will stop being valid in 2010 or so. The only way to get stickers is to buy a used hybrid that already has them attached and apply to have them (and their special FasTrak unit) transfered to your name.
An a related note, cars powered by natural gas CAN still get HOV stickers. That includes the Civic GX, as well as a few Crown Vic taxis. From what I understand, there were never limits in place for the number of stickers they will issue for these cars.