Joe has a great essay on what 20% time is and why it works. Very much worth reading: Joe Beda's EightyPercent.net
I am often confronted by non-googler misconceptions about the nature of 20% time, from now on I'm just going to point them at this essay. #4 especially is worth reading. 20% time is really not all that optional :-)
One thing Joe mentions in #5 that I'd like to stress is that, at least for the Googlers I've interacted with, there isn't a lot of 'stop energy' at google. My colleagues in engineering, marketing, etc, don't react to new ideas with "You can't do that" , but usually with "cool" and a tip as to who to talk to in the organization who is likely to be of the most help. I don't present this as a "this is why other companies aren't going to 'get' 20% time, but so that other companies can do this well.