It was a pretty big download taking about 10 minutes on the wireless connection to the house's broadband.
Now iTunes reports it's backup the iPhone. It appears to be much slower than a normal backup of the iPhone. In about 20 minutes of backup, it's progressed to what the progress bar suggests is about 5% or so. I suspect it might take all night. I'm glad it's the end of the day and I can just let the two gadgets work on it overnight.