This gives you a problem with TrueCrypt if you use the encrypted device method, because you can't put in your admin credentials because you don't have any.
What you need to do is to switch user accounts, and launch TrueCrypt as an admin. Start terminal and type:
Then TrueCrypt will behave just like you launched as the admin, remember when you select device to put in the admin account's password.
The terminal session will stay showing that you launched TrueCrypt, which is fine. It'll stay that way until you quit TrueCrypt.
Update: much better way
- su -c "/Applications/TrueCrypt.app/Contents/MacOS/TrueCrypt &"
(line might be wrapped)
This will ask for the root/admin users password, launch TrueCrypt, and then put in the background, ie you get your terminal session back right away