What font-stack do you use for Lucida Grande?
Being a front end developer I have close connections with three groups of people. Designers, customers and back-end developers.
Mac Designer (Safari - isn't sure what version, "but why would that matter anyway?") wants Lucida Grande for the top navigation. The finished work has to look exactly like their mock-up on their Mac or you obviously don't know what you're doing.
PC Customer (IE6 - may have installed Safari at the suggestion of the designer) doesn't know what fonts they are looking at, but it doesn't quite look like the mock-up. Hopefully you were smart enough to level a couple of obvious mistakes for them to critique so that they don't notice the additional space on the last element of the top-navigation.
Ubuntu Developer (FF - but it's the *nix version, and they certainly don't have any fancy fonts installed on their box). The top-navigation wraps horribly onto two lines, but you were smart enough to use a background color that contrasts with the text, so they can still use the site. They don't care, the site is failing gracefully, but your pride is hurt!
The font stack I usually use is:
font-family: "Lucida Grande", "Lucida Sans Unicode", "Lucida Sans", Verdana, sans-serif;
What do you use?
Most people don't consider Linux users because most Linux users have already expressed their complete lack of interest in aesthetics and typography by way of choosing to run GNOME or KDE or whatever other alterna-window manager is popular these days.
Even if they have the font, the text is going to be rendered differently by different systems. If you want something that looks absolutely identical on every platform then you have to use images. Since you seem to be talking about a navigation bar here, that wouldn't be an unusual practice at all.
font-family: "Lucida Grande", "Lucida Sans Unicode", "Lucida Sans", Geneva, Verdana, sans-serif;
and a good place to check that is useful would be: