I dont' know. There could be several reasons. One, you are not already on a version of iOS 5.0. Two, It can't not connect to the OTA server (for unknown reasons). Third, some esoteric reason that defies explanation.
At any rate, my main point is that if you are getting an OTA update you don't connect to the computer at all. That's what OTA means. It also means you get a smaller (delta) update that only contains the changes from the older iOS version. If you connected to the computer you got a full update via iTunes (contains the entire new iOS system file); and you need the most up to date version of iTunes to do that.
Agreed. My laptop iTunes is at least a generation out of date, but I updated four iDevices OTA without prob. I never even turned on my laptop; it was irrelevant.