I find it amusing that anybody espousing web standards would complain about Safari; it's not perfect, but being that it does pretty darned well with W3C specs, there's little if anything to complain about. Assuming that the site isn't using a very broken browser sniffer (which sounds like what tace is dealing with), so long as it validates and looks right in Mozilla or Opera (or any other decent standards-compliant browser), it'll look fine in Safari, too.
I admittedly don't do much tricky stuff, but in my experience if I'm writing for the Mozilla engine, and the layout looks fine, I can pretty much invariably have it look almost exactly the same in Safari, since it's actually using standards unlike the disaster that is IE.
Safari's only big weak point is it's mimicking of IE's behavior on broken markup, which is a tragic reality of the web today; if the site is coded right, render it right. If it's coded wrong, render it wrong, but very specifically wrong in the same way IE does it, otherwise all those badly written sites that "look just fine in IE" won't look just fine in your browser.
Incidentally, tace, if you're not trying to come across as sarcastically biased against Safari, you're doing a good job of it anyway. Your point about good design meaning that it just works out of the box is a good one, but misses the fundamental point that the average user doesn't use tabs, and if the average users I know are any indication, would be thoroughly confused by them--most still double click on web links, for heaven's sake.
Since it takes exactly one menu selection and two clicks into the very simple pref window to enable tabs, and 95% of the people who are interested in using tabs are computer competent enough to take a peek at their preferences anyway (to set their homepage and such), not enabling them by default seems like a smart move to me. The simpler the better, with thanks for making the preference an option at all.
And by the way, the point those guys were making is that Safari is dominant on the Mac (at least, the OSX using portion of us), so if you're interested in supporting the Mac you should make an effort to support Safari as well, which should be painless due to the standards compliance. It's not a requirement, and if you don't want to make any special concessions for the small market that is the Mac, then you can safely ignore Safari, but if you're specifically targeting the Mac a lot of people use it.