In the font used for usernames, the X is oddly distinctly larger than the other letters (see: my username on answers here and on the main site). Not just a little, like it seems like a half-capital compared to the rest.
I'm experiencing the same effect (both with Chrome/Windows 7 and IE9/Windows 7).
It's very weird. the over-sized x appears only on HTML elements using the Lusitana Regular font. However, small caps letters of said font seem to be all in line with one another.
If your computer/browser doesn't have said font installed (mine doesn't), the CSS instructs the browser to use the Palatino Linotype which, again, has a "regular" small caps x.
I didn't experimented further; but if I have to bet on something, I'd say that the Lusitana font stored on the stackexchange server is not the regular version.