OpenConnect is known to work, with both IPv6 and Legacy IP, on Linux (including Android), OpenBSD, FreeBSD (including Debian GNU/kFreeBSD), NetBSD, DragonFly BSD, OpenIndiana/OpenSolaris, Solaris 10/11 and Mac OS X platforms, and should be trivially portable to any other platform supporting TUN/TAPdevices and on which OpenSSL runs.
For Solaris support, and for IPv6 on any platform, the vpnc-script shipped with vpnc itself (as of v0.5.3) is not sufficient. It is necessary to use the script from my vpnc-scripts repository instead.
It is known to work on at least i386, x86_64, PowerPC and MIPS processors, and should not have issues with portability to other CPUs.
Note that 'Cisco Secure Desktop' support may require the ability to run Linux/i386 binaries; see the CSD page.
Platform support for new UNIX systems is relatively simple to add — most of the difference is in the TUN/TAP device handling, and the major variants of that are already supported.
A port to Windows should be fairly simple, since a TUN/TAP driver exists for Windows and Cygwin should make the basic port work OK.
A port to Symbian, to provide VPN connectivity on phone handsets, would be very useful. Any volunteers?