In February of this year, Microsoft delivered KB2956128. KB2956128 is an update for Outlook 2010 to fix problems introduced in earlier fixes. This one in particular was meant to repair issues with public folder searching, configuration management, RTF file compatibility, memory leaks in Windows Explorer, proxy connection failures, and clipped dialog boxes for some languages.
In the last day, Adam Fowler has identified a new problem that KB2956128 has introduced. When installed, users can't change between mail profiles and sometimes profiles will disappear.
Adam says that uninstalling KB2956128 fixes this particular issue, but of course, this also means those problems the patch was supposed to repair will come back.
We'll have to wait to see if Microsoft will offer an improved patch soon, but if you're experiencing profile strangeness, it's good to know there's a culprit.