In this blog post, Colin Eberhardt describes a simple method for automatically showing the full text as a tooltip whenever the text in a TextBlock is trimmed.
Source: ScottLogic
The TextTrimming feature of SL4 allows us to worry a little less about the layout of our applications, giving the option to use fixed widths when rendering text. This can be particularly useful if you have to localize your application. However, arbitrarily trimming the text without providing the user with a mechanism to see the full text is not great for the user. A simple solution is to use a tooltip to present the full text.