Same problem here.
2011 MBP 17" 2.2Ghz i7 8GB.
SL gave a reliable 5-6 hours off a full charge, sometimes more.
After Lion upgrade I was getting 3 max.
Did an SMC reset and on the first charge got back to 5 hours actual usage time.
Recharged (no shutdown), disconnected the charger, showed 7+ hours. Used the machine for a couple of minutes and the projection fell back to around 3.5 hours.
Now, after a full charge, I see a max of around 4 and frequently closer to 3 hours.
I am charging now and when complete I will see what the projection is. I will then try another SMC reset and see what happens.
For those concerned about Apple's unreponsiveness on this (an aside: my colleague was told by Apple Care that 3 hours battery life with Lion on a 2011 MBP was normal becuase of the fast processor -- when he responded that he got 6 hours with SL on the same machine, the tech had no response), I will opine the following:
Everyone I know with an MBP running Lion has this issue.
There has to be hundreds if not thousands of Apple employees, inlcuding many senior ones, running Lion on new MBP's and they have to be seeing the same issue, so there is no way that Apple can be unaware of this.
As usuaul they won't acknowledge it as they don't want the sensationalist headlines, but I bet they are working on a fix. We just need to have patience, frustrating though it is...