Tomas Restrepo led me to the solution to my ASP.NET 1.0 problem in IM tonight (see? it’s already paying off). The essential issue for me was one of security: the default settings for IIS 6 do NOT allow you to run un-approved ISAPI DLLs. There’s a Microsoft support article that sort of describes the process in a generic sense, but I thought I’d tell you what I had to do explicitly.
When you run aspnet_regiis for ASP.NET 1.1, it adds an entry to the Web Service Extensions system in IIS 6 (which is reachable from the IIS MMC snap-ip). ASP.NET 1.0, on the other hand, does not. In order to run ASP.NET 1.0, you need to explicitly add a new Web Service Extension, mapped to the ASP.NET 1.0 aspnet_isapi.dll file, and set the status to “Allowed”. Once you’ve done that, then you can enable ASP.NET 1.0 or 1.1 for any web applications that you wish using the respective aspnet_regiis tools (or by hand, like I did).