the extremely busy servers timing out
Things were quite busy yes.
The graph below is not from the download site, but from downstream somwhere in europe. See the lonely spike on the graph on the Wednesday ? That's the IOS5 downloaders doing their stuff... imagine multiplying this by X locations and you can only guess how busy Apple (or more likely Akamai) was !
(For people visiting this thread in a month's / year's etc. time, the above graph will be irrelevant as it is a weekly graph and so will have been refreshed .... so if you visit this thread after Week 41 of 2011, you won't understand what I'm on about !).