I have 10.4, not 10.5, but just to make sure you know about this stuff:
Open System Preferences|Network...
* Under the Show pop-up menu near the top, VPN (PPTP) is one of the
options and you can specifically set your DNS server address there.
* Under the Show pop-up menu near the top, select Network Port
Configurations and note the message about dragging items to put them
in whatever order you want.
In my case I have VPN (PPTP) first in the list so that whenever the
VPN is connected that DNS server takes precedence.