This has never been a problem before, but after the latest internal build (September 4) it seems that when setting a Width on RadComboBox, that width is not automatically applied to its items, and the clickable area of the items is too small. I've uploaded a screen shot of the problem (imagine that the mouse cursor is over the "LDAP" item):
http://cid-cc43a90a79374ebd.skydrive.live.com/self.aspx/Public/screenshot^_radcombobox^_problem.png
http://cid-cc43a90a79374ebd.skydrive.live.com/self.aspx/Public/screenshot^_radcombobox^_problem.png