If you've seen screenshots or played with the emulators then you'll know that a few (sadly not all) of the controls have been skinned with a new look. Specifically scrollbars and tab controls have a new look and one that can be customised using themes. These are now drawn using bitmaps so a theme can change the look and feel of the tab controls, message boxes and scrollbars beyond just changing the colours. If you run a .NETCF application on WM6.5 you'll find that the Tab Control is painted in the "old" way and does not take on the same appearance. I had a dig around and found that this was due to slightly different window styles applied to the native control. I put some code together to change the styles and lo and behold it worked (some styles can only be set when you create a window and cannot be later modified). The attached file is a source listing in C# which will apply this change to a tab control. Since this workaround is not documented it may break or change the behaviour in future updates so realise that this is provided as-is.
It is a shame that only a handful of controls have been updated, this actually makes some screens look odd because they have a mixture of curvy controls and old fashioned buttons etc - it would have been nicer to see a consistent approach across all the standard controls and ideally one where existing .NETCF code would get the benefit of any improvements without having to hack!