This is one of those “duh” type moments, but I just realized that, even though SP2 installs HTTP.SYS, it stil uses IIS 5.1, which means IIS is NOT using HTTP.SYS.  This would explain my recent trouble and why trying to control port usage with the httpcfg tool didn't seem to do anything.

I'm pretty bummed about this. I probably would have installed 2003 server instead of XP had I realized this sooner.  I'm going to give a serious effort to write a managed Subversion server that uses HTTP.SYS.  That would probably reduce the barriers for using Subversion sufficiently to get more people interested in it.

[UPDATE]  In case you didn't make the leap and realize what the implications are for IIS not using HTTP.SYS, it means you can listen on the same port that IIS is.