Thanks for bringing our atention to the issue.
Currently in Silverlight there is no auto-clipping feature (as in WPF) and we created this control that tries to do just this. Unfortunately every TextClipper control invalidates the layout at least once (i.e. is measured twice) because there is no way of knowing the size of the text before it has been rendered.
I suppose that this can be implemented naively, assuming an average width for the font characters depending on their size. This may save a few layout cycles and prevent the error. This of course would mean that the clipping will not be very accurate because in most fonts there is width difference between characters, say 'i' and 'W'.
The reason why you get the error around 250 elements is because the you cannot invalidate the layout more than 256 times in a single pass.
If you are interested in the "naive" TextClipper, please come back to us and I will prepare an example with sample implementation.
Thanks again for your feedback,
the Telerik team