Version: (using KDE 4.3.0) Installed from: SuSE RPMs It is not usual that user works most of the time or all the time with only one tab in messages tabbar. In such case tabbar takes space with 0 benefit for user -- please provide such option "hide tabbar for only one tab" as it is present in Konqueror. For lowres screen users it is extremely useful.
preferences/Appearance/Message List/ "Hide tab bar when only one tab is open" is what you're looking for.