MAC address = Media Access Control address
Believe I saw that this was possible somewhere, but now that I'm looking for it I can't find the implementation.
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The MAC address is definitely not something you can get through a web portal unless a bug exists.
If you can get someones MAC address you can change you own through some hardware/software tricks and thus make your computer look like theirs which is a very bad thing for security!
I really think this would be a security issue if you can see it with JS or Flash. Actually, neither JS or Flash shouldn't offer access to any details of the user's PC except some very basic data about the OS, browser, screen resolution and such, things that you would actually need quite often. Who knows, might be possible with IE6 :D