After updating the infragistics controls to the latest version (from 2005), the space to the left and right (and behind) of any tabs in the control does not appear as transparent. I am not sure where the color is coming from, but is there a way to change this?
Is your application now using AppStylist ".isl"? It might be, since AppStylist was introduced in 2006 Volume 2, is now causing this behavior.
Note, you may want to simply clear out any TabHeaderAreaAppearance applied to the UltraTab or set UseOSThemes on the control to figure out if the issue is caused from system settings.
Let me know if you have any questions.