O.K. as said i stopped using "sudo" a long time ago. but as far i can tell from here;: sudo is sudo is sudo !
@magnetic:
If he is able to mount the partition from sudo then he is able to do anything from sudo !? It just depends on what sudo accepts to be anything !
Your being not able to make Ubuntu accept your password is based on the fact that your user was not made a "sudoer" !?
@Acill
1. Never ever mount anything on "/mnt" directly ! Never ever ! ! !
That's bad ! mKay ?

Use "/mnt/$ANYTHING_HERE" instead
2. Bad syntax, try adding the boot partition to your fstab with the "noauto" and perhaps the "users" option.
3. try to edit the config file with editors like "nano". to avoid any problems arising from graphical-tools use.
If it still does not work buy yourself a punching ball and wait for advanced help from people knowing Dapper !?
best wishes,
H.
P.S.: this does not mean i am not willing to try to help, but i do not know if you really have the time discussing this problem any longer !?