iOS 5 became available for download around 6pm GMT on Wednesday evening, however since then a large number of users looking to upgrade at first opportunity have found themselves frustrated in the process. The problem appears to be relating to excessive server dmand at Apple.

It's likely that such large numbers of users are attempting to upgrade at the same time that it is causing large server spikes. While downloading of iOS 5 itself appears to work fine, during the upgrade process itself, iTunes attempts to verify the download with Apple at which point it terminates with an error.

Hopefully this should ease over the next few hours.