Try the following: Reset a raw image in LR4 to adobe defaults (PV 2012), then set clarity to maximum and edit in photoshop (with ACR 6.7 installed) using photoshop itself to render the image.
Result: The edited image (with no changes made in PS) appears a lot brighter.
When rendering with lightroom, it looks exactly the same (as it is expected).
So, something is wrong with ACR 6.7 here. If one does not use clarity, but the other new tonal controls, there are similar effects, however not as severe as with clarity.
I use Win7 64 Bit, LR 4.0 and PS CS 5.1 (12.1), both 64 Bit, and ACR 6.7 RC.
Edit: Here are two sample images. First the one that was edited using "render with lightroom" - it appears exactly as the original edited image (with clarity +100):
And here the same image edited in PS CS 5.1 ACR 6.7 RC with "open anyway", i.e. rendered using ACR 6.7 RC: