I think there are two levels of culture-jamming. There's the serious, and there's the just for fun. I prefer the second, as I doubt the first will ever have any effect.
For some great examples of the second version, try
Improv Everywhere (they were just in the news for No Pants 2K8), as well as the audio-only
Free Speech For Sale (part of the Snuggles collective, inspired by Negativland).