I can fully confirm that this problem doesn't show up with apache 1.3.27, just with 1.3.28. Reverting back to the previous version solved the issue.
Thanks to everyone for the advice!
I can also confirm this on Linux with kernel 2.4.20.
It appears to be a general problem with cgi execution and apache 1.3.28 as perl cgi-scripts also tend to get stuck as zombies (and don't with 1.3.27).
Looks like the Apache developers are aware of this issue: http://www.mail-archive.com/[EMAIL PROTECTED]/msg17169.html
Looks like it's fixed in CVS and thus in a forthcoming 1.3.29
