I lost my last piece of valid ID the weekend before thanksgiving, and so got stuck flying without ID to and from Texas to see my grandparents. I was nervous before hand, so I shoved every piece of offical and unoffical documentation I had in my luggage. I had photocopies of the lost ids, my social security card, my birth certificate, expired passports and miltary ids... basically everything.
When I went to get on the flight at SFO in San Francisco they looked at my checkcard which has a very fuzzy picture on it from high school and my miltary id which expired 14 years ago. They guy checking ID said it was better than just a driver's license and I didn't get any extra screening.
On the way back from Texas I ran into a little bit more trouble. The lady checking IDs told me I really should have returned the expired ID to the government and requested my health insurance card in addition to the check card and military ID. After that I was pulled aside for additional special screening. Talking with the lady doing the screening however I found out I'd gotten the new enhanced pat down due to wearing a floor length skirt, and not due to any issues with ID. As a note the lady doing the pat down was suprisingly profesional and seemed somwhat embarsed to have to do this to people. Quite a neat lady in general.
In conclusion: Flying without valid government issued ID is not an issue, though the standards are inconsistant. Skirts are much more of an issue.