reno air was pretty cool about this. i dunno if they still do it.
when i was flying to the bay area once (from seattle) i had a seat way in the back. reno has this deal where, if there's room at the time of the flight, you pay $30 and you can get a first class seat.
that's the one and only time i've flown first class. oh, and i'm seriously cheap about hotels thanks to priceline.
(i pay less than 25% hotel list prices).