You really should try to keep all the mysql related packages consistent - install them from ubuntu repo OR from Percona one.
Right now you have conflict between ubuntu ones (5.0.67-0ubuntu6.1) and Percona (5.0.91-percona-b22.intrepid.5).
I resolved that by setting the Percona repo higher priority.
To achieve this you create a file, i.e.:
/etc/apt/preferences.d/percona
and put this inside:
Package: *
Pin: release l=percona
Pin-Priority: 700
Then do apt-get update before you update packages.
In worst case you may need to do dpkg --purge on some broken packages.
(Didn’t test it on intrepid, but worked for me on karmic)