After installing, I found that the driver for my Canon MP970 had been zapped. I had to reinstall it.

Looking at what got installed (using lsbom from the command-line), it appears that this update addresses much more than just the Pro900/9500: it contains drivers for dozens of Canon bubble-jet printers and multi-function devices -- except not the MP970. Maybe it's too new. But it's possible that other devices' drivers are omitted and will be zapped too.

Further sniffing around in a Time Machine back-up shows that very many files have changed between the Leopard release and this update. So it really does look as though this is a general update for all bubble-jets. Except those it misses...