I doubt AS3x could compile under Cygwin without major PIA, though. Cygwin Tcl is a very-very much a hack: I don't remember the details but it is sort of a mix of Windows and UNIX stuff (i.e. it is impossible to compile Tcl distribution tarball of any release under Cygwin, let alone compile it with threads).
I think PG for windows is in beta at this point.
Windows Services for UNIX was recently discussed on the AOLserver mailing list. I don't think anyone has managed to get the thread-enabled Tcl compiled on it yet.
I think PG 7.5 is not officially in beta yet, though the PG sources now compile natively on Windows, and nightly binary builds are available:
I tried an OpenACS install with a nightly build a few weeks ago but it wouldn't work out of the box and I haven't had time to investigate. I do remember thinking that the error looked more like a 7.4 -> 7.5 incompatibility than a Windows issue.