If you are using HTML templates with SimpleTAL it will pass through HTML comments (previous to version 2.2 this was not true.) However, the same cannot be said of XML templates. In version 3.8 of SimpleTAL, support was introduced for passing through HTML comments but only if you have PyXML installed!
This seriously tripped me up one night. My pages were not rendering correctly in IE6 on my production site but they were rendering just fine on my test site. I went through the typical gamut of suspects like incorrect CSS files being deployed, UNIX file permissions, mismatched code etc.
Then I did a diff between the two sites from the view source output from my browser and noticed that the following magic lines were missing:
<![if lte IE 6]>
<link href="/css/ie6.css" rel="stylesheet" type="text/css" media="screen" />
<![endif]-->
Because the conditional is wrapped in a comment, simpletal was stripping out the whole statement and leaving IE6 swinging in the wind!
It started to get really ugly when I was writing scripts to cksum every damn python file to figure out who the culprit was. I was just about to jump through a window when no differences turned up in the library files. Of course google saved me in the end but there was a serious lack of hits on something I would think has tripped up many a developer.
UPDATE
I have noticed that on a Mac with OS X 10.6.2 (Snow Leopard) running python 2.6 in a virtual environment, if you install PyXML through easy_install you will pick up version 0.8.4 and it will be installed under the package name _xmlplus. Of course SimpleTAL will be looking for the package xml and you will be screwed. As a quick work around I hacked simpleTAL.py to look for the correct package. Just grep for references to PyXML and you will find the lines to change.