It may be an issue with their servers when detecting 10.7.x systems. I just bought a MacMini (newest one, with Thunderbolt) yesterday, updated iTunes and all other updates it recommended yesterday and wasn't ready to update 10.7 to 10.7.1, and went to do so just now and got the same error you reported.
I also have a MBP running 10.7.1 and it too won't check for updates, same error. It seems unlikely that two different comptuers on my network both have corrupted files.
That said, a Macbook running 10.6.8 can reach the server. Also tells me my router (AirPort Extreme BaseStation) isn't blocking the connection.