How to enter single user mode when rEFIt is installed?
"When you have rEFIt installed and you apply the shortcut to boot in single user mode (⌘+S), you actually land in the EFI shell.
In order to avoid that, boot as usual.
Then choose the Apple icon and press F2.
You will be presented with a series of alternate options to boot Mac OS X (e.g. single user mode, verbose mode, etc.)"

http://tateric.blogspot.com/2010/11/boot-in-single-user-mode-despite-refit.html

Note: After selecting the Apple icon with the cursor, do not press Enter. Right there, on the rEFIt screen, you hit F2. That will bring you to the alternate options, including "Boot Mac OS X in single user mode."