I hope this is the right forum to post this on, as my comments involve issues around the interaction between iTunes & iCloud, with an iPad2 (iOS5) and an iPhone 3G (iOS4.2). I trust that an administrator will move this post, if necessary.
I had used iTunes on Windows through several version revisions (up to the current v10.2, I think) to back up and sync an iPhone 3G (iOS4). When I got an iPad2, and upgraded it to iOS5 with iCloud, I enabled iCloud on the PC to permit wireless synchronising of the iPad - a useful benefit, I thought, even though I quickly became aware that the same enhancement was never going to be provided for the iPhone 3G I had.
However, I began to experience the failure of iTunes to start backing up my iPhone, even when the iPad was not involved - iTunes would start the iPhone Sync, but stall at 'Starting backup', and never progressing any further.
I forget where I got the clue from (I don't think it was my own inspiration), but just to see if it would help, I de-installed iCloud from the PC, and was still able to sync and backup the iPad with the USB connection.
Lo and behold, the sync and backup of the iPhone started working again - I have not had a problem since.
It appears that something in iCloud interferes with the backing up of an iOS4 iPhone device, with iTunes.
I mention this in case anyone else has noticed the same or a similar problem, and is in need of a resolution.