The conservative sites seem to be suspicious of the bin Laden video as well.

Hot Air: Digital forensic analyst: Osama video was manipulated

Rightwing Nuthouse: OSAMA VIDEO MAY BE A FAKE

Michael Ledeen in the National Review:

Third, is it really Osama? As you know, I was reliably told something like two years ago that Osama had died. Nothing in this speech sounds at all like the "old" OBL. That man knew how to give a stemwinder, he used elegant language, his threats were blood-curdling, his calls to the faithful inspiring. This man talks like, well, a high school dropout. In fact it reads like an "Onion" spoof. And the sound is bizarre, at least on my IBM desktop. It sounds almost as if there was enough garble in it to make it difficult to match with voice prints of the "real" guy. I'm not convinced.