You are using of course LTS for your servers, right? Because otherwise it would be stupid... So, is 3 years support not enough for you?
Actually, Ubuntu LTS server is five years. http://en.wikipedia.org/wiki/Ubuntu_(operating_system) In fact, 6.06LTS server support is just wrapping up this June. Desktop versions of LTS are three years.
I have a theory that it's impossible to prove anything, but I can't prove it.