[iPhone] Error backing up iPhone for iOS5?

So the iOS5 came out and I wanted to download it for my iPhone 3GS. So I updated iTunes and downloaded the update software. So it wants to back up my phone before the update and in the middle of backing up my phone, a error message pops up that says:
“An error occurred while backing up this iPhone (-5000). Would you like to continue to update this iPhone? Continuing will result in the loss of all contents on this iPhone.” I don’t what to do and I don’t want to lose everything on my phone lol.