Tcl 8.6 (Oct-2010: but currently not any official release) has IPv6 socket support built in.
Unix: http://www.unnumbered.net/~umeno/etc/tclIPv6patch.html also ceptcl provides ipv6 among other protocols.
Windows: http://sf.net/projects/iocpsock
This [L1 ] thread hints at a couple of the challenges in getting Tcl's high-level socket programming interface right.
Stu 2009-06-17 I noticed that Ceptcl wasn't mentioned in that thread, I wonder why. I'd post this to the thread itself but my 'net conn's sporadic lately.
Steve Bennett 22 Oct 2010 notes that Jim Tcl has support for IPv6 and udp sockets.