it is possible to download and save the file using Software Update.
Couple of problems there though. The iMac will be getting some different software updates from the MBP's.
The OS updates should be fine though.
Another way is to go to the Apple site and download the installers from there. They are self contained and will run very well offline from a DVD or CD.
I handle all the maintenance for three macs. My own mac Pro, my wife's Macbook Pro and my mother's iMac, all intel and purchased within the last year. All are running OSX 10.5.8, except my mother's which is at 10.5.5. My mother lives in another state and she only has dialup. Downloading a 60MB file takes all night when I'm there. Last update I did took me a week to download and I don't like tying my mother's phone up all night in case she needs to call an ambulance. Is there a way I can download updates using software update and my cable modem to my Mac Pro and then burn them to DVD and install them on her machine?