Pownce apparently opened up to the public last night at midnight. TechCrunch gave it an enthusiastic write up when it first went to beta, and again yesterday. The comment stream on TechCrunch was less kind, I'm afraid.
Think of Pownce as twitter + file and event sharing. At the moment, Facebook serves this purpose for me. I'm planning to be a Pownce laggard, as a result, rather than an early adopter.