problem updating to ios4

I'm trying to update my ipad to ios4 (and I'd like to do the same for my iphone even though its only a 3g!)

When I go to itunes with my ipad (and with my iphone exactly the same thing happens) and press check for update on the device summary page I get the message - itunes could not connect to the ipad software update server because you are not connected to the Internet - when my pc is definitely connected to the internet! And itunes "knows" that for all purposes including accessing the itunes store etc but I can't get past the error message when I try to upgrade my ipad os.

I've updated itunes to 9.2. Doesn't make any difference!

Any ideas?!