I mean that Gmail uses ajax (that fancy schmancy group of technologies - xhtml, javascript, xmlhttprequest...- that lets you incrementally update the page) for everything they do, and despite there being standards, there are certain things that are not cross-browser compatible. Same goes to most of the web 2.0/ajax web applications out there that aren't compatible with safari. changing the user agent isn't going to do anything because safari is not going to render the page the same way as firefox or internet explorer regardless of whether or not safari pretends that it's MSIE or firefox which is all changing the user agent does (which might get you past lame javascripts and pages telling you that it's in your best interest to use MSIE6...but not for this)
so in essence it takes them more effort to work on a safari-compatible version of gmail chat because the code that works on IE doesn't work on Firefox, and when you get it to run on Firefox, you discover that it also doesn't run on Opera and Safari, etc., which just means more work to get the stupid thing working on all browsers correctly because what works in one (like back-button functionality) is broken in another. Let's not forget browsers that can't handle it at all, then you have to worry about making it degrade nicely and telling the user to go stuff it because you're not going to support x browser for the time being..
btw, i dont think ANY browser supports all standards correctly
Especially not CSS, which is why I hate it so much
Safari's better than most browsers when it comes to the Acid2 test, but that doesn't mean crap if everyone else (read: firefox, msie, opera, ...basically you have 98% of all people using browsers right there) doesn't follow the standards as well. I'm sure the people over at w3c know that very, very well