When I changed my discs I had to buy one of these:

(It's an Irwin bolt grip)
As the previous owner / mechanic rounded off the caliper bolts.
When reinstalling calipers you should use some loctite 243 on the caplier to car bolts, and possibly others if you've broken their seal, I took my calipers off in a different way to the how to as I wasn't changing pads. Quote from Loctite:
"LOCTITE® 243 is designed for the locking and sealing of threaded fasteners which require normal disassembly with standard hand tools. The product cures when confined in the absence of air between close fitting metal surfaces and prevents loosening and leakage from shock and vibration. Particularly suitable for applications on less active substrates such as stainless steel and plated surfaces, where disassembly with hand tools is required for servicing. The thixotropic nature of LOCTITE® 243 reduces the migration of liquid product after application to the substrate."
This post has been edited by SteveHall: Jan 29, 2008 - 6:36 AM