Alright, here's some screenshots (click on them for full size so you can see the anti-aliasing effects used on the text):

 

This shows that the regular UI components (ribbon, favorites/folder list) do not use ClearType, as the anti-aliasing in that image is clearly grayscale. Note that this is the case in every Office 2013 program, not just Outlook (the ribbon in e.g. Word also does not use ClearType).

This shows that the inbox message list does use ClearType, as you can tell by the coloured fringe of the text (ignore the Japanese text; MS Gothic does not support anti-aliasing and I've not yet figured out a way to instruct Office to use Meiryo in the UI).

 

Finally, this shows that while the message header does use ClearType, the message text itself does not. This is to be expected since I believe Outlook uses Word to render messages, and Word also does not use ClearType for document text.

This behaviour is unique to Office 2013. All of these UI components in all Office programs used ClearType in Office 2010.

I have this problem on two different machines: my desktop (ATI Radeon HD6850) and my Sony Vaio Z laptop (hybrid Intel GMA / nVidia GT 330M; the issue occurs using either card). Both are running Windows 8 with Office 2013. Both were clean installs (not upgraded from Windows 7, and they never had any Office version other than 2013 installed either).

Charles, if you are truly not seeing this problem I would like you to post a screenshot of these same UI elements (also under Windows 8) to prove that they are supposed to have ClearType. Because frankly, I doubt it.

EDIT: I tried disabling hardware acceleration, and it makes no difference.