Since Q2 2010 all new functionalities are listed in the control's Release Notes posted here.
New Skin - Windows7
New Skin - Sitefinity.
Performance optimizations on the server - by deleting reflection code we have achieved up to 30% improvements depending on the scenario
Support for Visual Studio 2010 Beta 2
Scrolling for vertical RadTabStrip
Added a OuterCssClass property to RadTab (gets added to LI elements)
Bug-fixes
No new features added
Added new server side methods FindTabByText, FindTabByValue which have a second argument ignoreCase which indicates whether the comparison is case-sensitive (true) or case-insensitive (false).
Added a new server side method FindTab which has one argument match of type Predicate Generic Delegate. This method can be used to find an Item by some custom criteria.
Added protected virtual methods for server-side events – OnTabDataBound, OnTabClick, etc.
Support for server-side expressions (<%= %>) in the TabTemplate.
Hierarchical databinding to IEnumerable via the DataFieldID/DataFieldParentID properties
findTabByUrl and findTabByAbsoluteUrl client side methods of RadTabStrip
Added ClientChanges property to the RadTabStrip class. Now client-side changes are available on the server side after postback.
Added findControl client-side method of RadTab class.