Unknown error-23 while downloading iOS 5 iPhone 4

So I have been using the iPhone 4 for nearly a year and a half now with no issues whatsoever.

I wanted to update to the iOS 5 earlier today. When iTunes tried to download the OS, it failed with error -23, which is an unknown error. The rest of the message is a cryptic "there was a problem downloading the software for the iPhone <name>".

I went ahead and got the ipsw file for the GM release of iOS 5 anyway, and proceeded to install it using the "Restore" option (I kept Alt pressed on my keyboard and pointed iTunes to the ipsw file).

I now have iOS 5 on my phone and it works well. However, all my contacts have disappeared! iTunes has restored my music and my apps, but not my contacts. When I try to use the restore option and get my contacts back from an earlier back up, I experience the same error -23.

What worries me is that I haven't found a single relevant post on the web related to the specific error code "-23". I have no idea what the issue is and how I can correct it now.

Any help on this front will be greatly appreciated.

FYI,

iTunes: 10.5 (141)

Mac OSX: 10.6.8 (I gather that the latest version is 10.7.2, but I don't see this update in my software updates...is this an issue?).

I am from Singapore and I have a Singapore Apple ID. Please reach me if you want any more information.