not sure how it works either, but it works (i just tried it). seems that if you run regsvrex as a user (with the /c switch), it creates all the COM keys under HKCU\Software\Classes.

Then, these HKCU keys are "merged" into the "HKEY_CLASSES_ROOT" and appear just like they would have, if an admin had installed them there.

Very slick.

-Shawn