Update 3/27/06: Well, after only a couple of days, I'm sold! This great little tool does precisely what I hoped for, and more! Though it's a little fussy on this point, Observation Post lets you connect to Bonjour services through its interface as well as getting notified about them. It tops HardwareGrowler by not only displaying a wider range of Growl event notices, but the notices also carry an icon identifying the kind of event. Very helpful, that.
All in all, a very satisfying little bit of freeware! If you are already addicted to Growl, I highly recommend Observation Post in place of HardwareGrowler.