I wrote a PHP script which works with Graphviz to create a graph of
recursive dependencies of a port. For example, here's what it does
with the gimp2 port:

http://www.ryandesign.com/tmp/gimp.png

I haven't made it available anywhere yet, but I can send it to you if
you like. Or if you tell me which port you're trying to install, I'll
show you its dependency graph.