Not always Silverlight TextBlock that contains long text has enough space for shows all this text. TextBlock clips text in this case - this behaviour isn't best way for this. Because I wait that TextBlock not only trims but also adds "..." to clipped text. WPF has brilliant property that helps to resolve this trouble - TextTrimming. I implemented this possibility for Silverlight and now want to present it for you. Furthermore I implemented ToolTip that shows trimmed text (only if text is trimmed!)Enjoy!
UPD: I have just updated performance and behaviour when width defined zero.