UPDATE (Jan 2007)Duncan McGrego has taken these scripts and run with them. There is an updated version on the cheeseshop! Thanks Duncan. (Behold open source!) end update
While attending FOSS.IN, one of the sessions was a website security session. I only caught the tail end, but it felt very similar to a session at OSCON this year by Nitesh. I also attended part of Rasmus' talk where he talked about a tool he uses for detecting XSS. Which in turn motivated (reminded) me to examine some of the python tools for programming and simulating web browsers. I checked out both Mechanize and Twill by writing scripts that convert TestGen4Web output to a python unittest and twill script respectively.
Both twill and mechanize seem to work pretty well and appear to be actively developed. I had some issues with twill saying "& was unepected at this time" (still haven't resolved that, though repeating the command doesn't cause a repeat of the error...) while running scripts. Twill appears to nicely ignore robots.txt (and possibly update the user agent?), whereas I needed to explicitly tell mechanize to both ignore robots.txt (as well as change the user agent to impersonate moz). That behavior seems reasonable since twill is meant to be interactive.
One area where both are (understandably) lacking is javascript support. With ajax becoming more popular and widespread, this functionality is likely needed. Though this is probably a case where a pure python solution would seem silly. Python wrappers to the moz object model seem the way to go (don't know if that's possible though).