I don't hear that fortunately. I hear "what is that," or "nice Mac." The cool thing is that the PBs are so close in form factor to the early MBPs that most people assume I have an MBP.
I even got a nice comment on my TiBook/400 about two years ago when I was using it.
I don't worry about being out of date. My Macs get noticed because of their size and the fact that I am often pulling both of them out wherever I am.
Well, I think it's also because most people tend not to comment on my machines, so it might feel like the "your machine's older you should get a newer one" comments come more often than they do.
It did also startle people managing a surplus sale at my college that I was so interested in a "dead" dual 1.8 (2004) Power Mac G5 they had laying around - I guess they figured nobody would care much for an "old" machine. I ended up just getting the machine for a dollar, sticking in a new battery and DVD drive (though this wasn't really required to use the computer), and the machine ran just fine. Pretty much the biggest steal I've ever heard of on a computer, aside from getting something for free... (like my HP Pavilion ze5568 which I've outfitted with a light Linux distro, but that's kind of off topic.)
Said G5 hasn't seen that much use, unfortunately, although that's more because my workspace is so cramped and the G5 is just big enough to cause a bit of an issue. (Not to mention I'm a little worried about power use, but I'd have to do some more research.)