Windows
XP (32-bit), Vista (32- and 64-bit) & 2008 Server

AIX
5.2 & above

Solaris
10 and above on Sparc 

HP-UX
11i v2 & above (Itanium)

Mac OS X
10.5 & above (Intel Chip)
currently 4.6.1, 5.1 ships soon!

Linux
Kernel 2.4, 32 bit
Kernel 2.6, 32 bit