For a few years I have used NetBarrier as a software firewall with OS 8/9. Occasional scans at Shields Up! showed that my Macs were operating in stealth mode when I used NetBarrier.
So out of curiosity I went to Shields Up! and tested my iBook and 10.3 without a software firewall. The results surprised me: all ports except one were in stealth mode. The one exception was port 113 and it wasn't vulnerable according to what I read at Shields Up!
I'm not sure if this result is because of Panther/Unix or my Linksys router. Anybody out there want to hazard a guess?
At this point I don't see a reason to upgrade NetBarrier. It would seem that things are fine just the way they are...