Darren,
I have used a Corasworks module for top navigation that uses lists to control the entries and enable security trimming based on list visibility. It's quite easy to use and lets you set-up navigation without dipping into code yourself. It's probably only available as part of their suite but I'm sure there are similar webParts available out there.
Otherwise it's likely to be a coding job.
Paul.
www.myfatblog.co.ukMCTS: WSS 3.0
MCTS: MOSS 2007