Anyone tried the combination?  AOLServer can't (so far) compile under Cygwin, but has anyone tried to do that under SFU?

Regardless of version, really.  4 would, naturally, be nicer.

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 always recommend running AOLserver natively on Windows.

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.
http://search.gmane.org/search.php?query=Windows+Services+for+UNIX&group=gmane.comp.web.aolserver
http://article.gmane.org/gmane.comp.web.aolserver/10826/

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:
http://momjian.postgresql.org/main/writings/pgsql/win32.html
http://www.hagander.net/pgsql/win32snap/

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.