The download and installation of Xamarin Studio and all the supporting tools worked flawlessly on my MBP / OS X.
During installation, my antivirus flagged two of the userdata.img files (under SDK 12 and SDK 14, IIRC) as a Trojan.
I'm typing this from memory, but I think the AV (ClamXav) quarrantined the files after detecting ANDR.Trojan.GingerBreak.
I updated my AV preferences to ignore those two userdata.img files, then moved them back to their SDK homes.
Everything seems to be working just fine. Just curious if anyone else ran into something similar.