See the link below for source and what is more or less a toy applet to demonstrate sniffing the MAC Address(es) of a machine from the browser. The HTML illustrates the simplest (albeit hackish) approach for cross browser support. I've tested this with FF 2.x, IE 5-7 and Safari 3.x on Windows, unfortunately Leopard doesn't support Java 6 yet. This solution will only work with Java 6, I do no checking in the code for this fact, it's a demonstration after all.

One thing is for sure, IE treats Java as a third class citizen, I've done some timings: FireFox averages  10.5 seconds to start the Java plugin (which is barely acceptable), but IE is terrible with an average startup time of 31.8 seconds this is after clicking through the two levels of dialog about how this applet is insecure and may destroy your system, "oh the humanity" of course this can be turned off, but the default for most folks is to try and scare the pants of you.

You can find the package and source here:

http://www.softwaresamurai.com/Agwego/mac/macaddressapplet-1.0.tar.gz