This has happened after I open various windows, especially Tools windows, and it seems to happen a random times.
In this instance I opened the History window, and after the window was drawn Logos completely stopped responding. But if I bring another Windows app to the front and then bring back Logos, it works fine.
This time I checked the log files and I noticed that this error was being written over and over to the log:
2009-09-17 11:06:50.2358 1 Error TextBlockUtility TextBlockUtility.IsTextTrimmed breaks Text bindings; use TextBlockUtility.BindableText instead.