But it appears not 64-bit. From Cygwin FAQ:
Cygwin can be expected to run on all modern 32 bit versions of Windows, except Windows CE and Windows 95/98/Me. This includes, as of the time of writing this, Windows NT4, Windows 2000, Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008, Windows 7, as well as the WOW64 32 bit environment on released 64 bit versions of Windows (XP/2003/Vista/2008/7/2008 R2). As far as we know no one is working on a native 64 bit version of Cygwin.

Update: "part of the express versions of the Microsoft development environments". Those don't currently support 64-bit either.