I’ve written a lot about MathML support in various browsers. So let me just address your questions.
Safari may “luck out” and display a few mathematical symbols correctly, but it does not support MathML.
Gecko-based browsers (Mozilla, Firefox and Netscape 7 but not Camino) support MathML.
IE 6 with the MathPlayer 2.0 plugin supports MathML, too.
In all cases, you probably need some additional fonts installed: preferably, the Computer Modern (Postscript or TrueType) fonts that come with any modern TeX installation (or can be downloaded separately). An alternative, but much inferior font set is the Mathematica fonts. Unfortunately, the latter is the only alternative for MacOSX users, as the CM fonts are not ATSUI-compatible.
Eventually, someday, the STIX Fonts will be available, and will provide high quality mathematical fonts that will work on all platforms.
In addition to the inferior fonts for the Mac platform, the MathML rendering of Macintosh Mozilla sucks rocks. Mozilla for Linux and Windows is light-years ahead.
IMHO, as long as you stay away from the Macintosh platform, sophisticated math using MathML is ready now.
No one seems to be working on fixing Mac-Mozilla’s MathML support, so unless Dave Hyatt suprises us all with MathML support in Safari, the Macintosh will continue to suck as a platform for viewing MathML for the foreseable future.
Most everyone else, if they are willing to install some fonts and maybe a plugin (for IE 6) will be quite happy with the MathML support in current browsers.