Yep -- I do in fact have access to one external address enabled on my PIX, but I should have modified the line to show an internal interface before posting it here.

In general I recommend closely reading the PIX IOS docs in order to configure the PDM. Although it requires authentication just like telnet access does, any time you're in a position to enable administrative access of any kind you're at risk of enabled admin access to the wrong audience. Or at least, to an audience which doesn't require it.