You can already sniff for 'Mac OS X' (or more generically, 'Mac OS'). We already did this for 'Windows; '. The difference here is that the 'Windows' string appeared twice in the UA, whereas 'Macintosh' is unique. More potential for breakage here. This change would go more toward reducing header size (11 bytes) and consistency ('Windows' is gone, 'X11' should go too; leaving the same number of semicolon-delimited fields across the platforms).

Opera/9.80 (Macintosh; Intel Mac OS X; U; nl) Presto/2.6.30 Version/10.61 Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_4; en-US) AppleWebKit/534.3 (KHTML, like Gecko) Chrome/6.0.464.0 Safari/534.3 Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10_5_8; ja-jp) AppleWebKit/533.16 (KHTML, like Gecko) Version/5.0 Safari/533.16 Mozilla/4.0 (compatible; MSIE 5.5b1; Mac_PowerPC) (lol!) No precedent here, we'd be blazing the trail.

More concerted proposal in bug 589353.