OK, I went back and checked...
It looks like it DOES NOT work in 2008. The only reason it *seemed* to work is that in 2008, and evidently in the 2009 beta, Autodesk did not create an "IsPureAcadProfile" registry entry in the default C3D profile. In my code, if there was no entry, I assumed it was set to false, and everything worked.
Now, in 2009, they have added a registry entry with IsPureAcadProfile set to 0x00. When I try creating a similar entry in 2008, it blows up 2008 as well.
Well, once again, Autodesk has taken something simple and made it complicated. All I want to do is find out if the current application instance is running as Civil-3D or as "Civil-3D as Autocad". I thought the UserConfigurationManager was the easy way to do this, checking for the "IsPureAcadProfile" registry entry. But that entry is a DWORD, and the UserConfigurationManager evidently cannot read DWORDS.
Thanks again, Autodesk. But i've been working with Autodesk software long enough that I'm not surprised - I just need to follow the usual routine, and spend the next X hours trying to come up with a workaround, instead of working on what I wanted to do...