Well your machine is therefore unique <g>.
That is incorrect but sure, let’s see where that came from. Now way back in say 2008 (around LR2), there was a time when the previews may have been in ProPhoto, no longer.
I've thought about it some more and re-read your post, and I think we're talking at cross purposes.
Previews are Adobe RGB, no argument. That's because they're stored in jpeg format, and using a very wide colour space in 8-bit data isn't a good idea (you get large tonal steps).
However, in Develop Module, LR isn't using the previews. It's displaying the image data that it's editing, rendered from the ProPhoto RGB image data in real time. This is why there are sometimes differences between what's displayed in develop module and library. For example in LR3, sharpening and noise reduction were not always shown in develop module except at 1:1 zoom. This was to save processing time. In library module, processing time isn't an issue as what is being displayed is a pre-prepared preview.
Or are you saying that in Develop Module, LR makes takes each edit you do, re-renders the image and updates the preview in real time each time you make any adjustment?
After each edit or movement of a slider, it converts the entire image to Adobe RGB, converts to jpeg, stores the preview, converts it back from jpeg, and colour-manages the preview to thedisplay? Wouldn't it be simpler to simply colour-manage the ProPhoto image to the display? And then update the preview only once, next time you access it in Library Module?
I think the latter is what happens.
Anyway, I've launched a thread in http://forums.adobe.com/community/lightroom
in the hope that someone from Adobe might settle the question.