As I am reviewing some code at the moment, I have been looking for this blog entry from Rob for a while, and just found it again: Building a control which holds content: CustomControl vs. MarkupSubclassing vs. UserControl. Included in my specification references, but posting it so I can reuse it as a reference.
And the ever so useful Neil (when are you getting a blog back?) points me to an article by Kevin on the same subject: Don't subclass a Panel, unless you're making a Panel.
It always amazes me that people, after x years of OOD, still don't see a problem in using inheritance instead of combination / encapsulation.
Then again these guidelines postdate most of the uses I've seen so far so you can't really blame anyone.