How credible are these rumors? It seems like it should be possible to confirm or deny them by either monitoring Skype network traffic (to see if it’s sending data to a third party) or by reverse-engineering the Skype binaries. It also seems like if the “back door” were made available to a significant fraction of the world’s governments, it would be a hard thing to keep secret.
On the other hand, the showdown I predicted has not yet occurred, so it’s conceivable that Skype reached some kind of accommodation with US and EU regulators and quietly pushed a back door out with new versions of the software.
Update: One Slashdot commenter points to This report from Black Hat on efforts to reverse-engineer Skype. Looks like they’ve gone out of their way to thwart both tactics. Everything’s encrypted, and the peer-to-peer architecture means that the client sometimes randomly transmits data when you’re not making calls.