[ndaiger]

An interesting example from the past is the LiveJournal "Express Lane":

http://www.livejournal.com/support/faqbrowse.bml?faqid=74

I wasn't a LiveJournal user, but saw one of Brad Fitzpatrick's excellent scalability talks years ago. I think he had a special flag that could be set for paid requests that would give them priority in perlbal, the load balancer he wrote.

Obviously it doesn't do much when something big is broken, but Brad did have a way to make sure paying customers' requests were processed first, and had realtime stats on average request latency for paid vs. free accounts.