Freebase and Safari
Though Metaweb isn't supporting Safari at this point (as a dialog box will warn you before entering Freebase), Freebase actually is usable in Safari. The screenshot above shows Freebase running in Safari. I haven't tested it extensively but so far I haven't seen much more than a few layout quirks when running in Safari. So the developers just haven't had the time to completely support Safari and make sure everything works. I suspect that Safari support will be added in future versions (hey its only in alpha, give 'em a break).
Also a little myth that has been running around the web a bit is that you have to have a Freebase account to use Freebase. This is true for the alpha as they need to iron things out before public release but non-registered users will be able to use Freebase when it is ready. (This is according to the Freebase FAQ.)