Hi, two options I see...
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Reset Password from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Reset Password.
And though I haven't tried it on 10.6/10.7, I think this may get you in...
Reset OS X Password Without an OS X CD...
http://theappleblog.com/2008/06/22/reset-os-x-password-without-an-os-x-cd/
Admin Hack...
http://www.hackmac.org/?q=node/4
Starts up like the first time you buy a new Mac, but after filling in all that info again, you should have access to the computer and the other Users & files will still be there... give the new User a different name than an existing one.
Of course neither of these will work if you have Firmware password protection in Mac OS X ...
http://support.apple.com/kb/HT1352
It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option, and Shift, as well as booting from anything but the Hard Drive.
Then again, we might need to Archive & Install to get Directory Services back.