I just submitted a ticket for this but thought I would try here as well.
We are using the 2010_1_0309 release and have tried both the suggested workarounds (explicitly setting SelectedIndex = 0 on the TabControl, setting IsSelected on the TabItem), and neither work.
1) We're considering usign the internal 2010_1_0326_Dev build. Will that resolve our problem?
2) What if we just used specific DLL containing RadTabControl, are there dependencies on all controls being from the same build?