So finally, I have "Tracker" working with "Zope" 2.5.1 and "Python" 2.1.3 on FreeBSD 4.5. It seems that the restricted Python engine (which I presume is new/revamped in 2.5?) has a problem with expressions that are heavy with parenthesis. So, after crawling through a lot of DTML code (some of it quite old - some of it predating the 'let' tag!) by slowly moving a 'dtml-raise' statement around, I was able to find all (or at least most) of my problems.
I just couldn't take "Roundup" any more. Actually, as a single developer or with another geek, it was alright. But it's still young, and doesn't have a lot of the great user/security/collaboration issues that Tracker has (and which CMF Collector partly has). And it doesn't have the workflow. And it's still too much tweak-work to configure ones own instance of "Roundup" beyond the two templates they have. I am going to try to stay active with it though, but it really feels like the goals of "Roundup" and "Tracker" are very different.
Tracker is an old unwieldy beast, however, and it seems to take a fair bit of grunt work to get it to work every time Zope has a new revision. Ugh.