I love my ergonomic MS Elite keyboard, so I use it on my Mac Pro workstation. However, today I needed to boot into single user mode to repair the filesystem of the boot partition. Normally you hold down the Command+S keys, but that only works with original Apple keyboards (and even then it seems to be unreliable for bluetooth keyboards). Apparently the key mapping happens in the drivers or some later stage of the OS and this is not loaded at the boot time.
I found out that the boot manager rEFIt has an option to boot n single user mode as well. Simply select the Mac OS X you want to boot and hit F2 to get a menu of different boot modes, such as safe mode or single user mode.