The reason TrueCrypt requires your administrator password is that it uses low level commands to create the proper devices and mount the filesystem and these commands in turn require root access (which the administrator acquires indirectly via sudo).
To avoid this, it's possible to use the "sudoers" file to give the right to a given user/application combination (in this case you/truecrypt) to automatically be run as root.
Log in as an administrator, then type the following command in a Terminal:
then add these two lines:
# Allow TrueCrypt run under user "XYZ" to run as root without asking for a password.
XYZ ALL=NOPASSWD: /Applications/TrueCrypt.app/Contents/MacOS/TrueCrypt --core-service
Of course, replace "XYZ" by your user account name.
Note that this opens a security hole since TrueCrypt will then have full access to any device you will mount your encrypted volume on. I have no idea how hard this would be to exploit though...
A few useful references: