"DMCA basically gives you 72 hours to take something down"
I don't believe there's any law or precedent that states 72 hours (or any number of hours) as a required turnaround time. More importantly, for the vast majority of infringing broadcasts, we don't take 72 hours (or any number of hours) to respond to a request for takedown. Instead, the vast majority of owners of infringing content (over 150 companies including probably all copyright owners you can name) have access to a tool that lets them take down content instantly without any intervention on our part.
"Livestreaming sites have put content fingerprinting technology in place and often respond very fast to takedown notices"
When you refer to these sites, I'm curious which ones you have in mind other than Justin.tv. We do have live finderprinting and we do respond instantly to most takedown noticed. However, I don't know of other live sites that are doing either of those. In fact, Ustream was sued this summer due to not responding to takedown notices and Fox's content group legal counsel has specifically stated that Livestream has not gone to the same measures to protect copyright holders as Justin.tv.
http://www.techcrunch.com/2009/08/17/ustream-sued-by-boxing-...
http://newteevee.com/2009/11/11/justin-tv-live-fingerprintin...
http://dl.dropbox.com/u/110031/JTV/Video/content%20filtering...