I've been working on this for the last couple hours. I placed the following code into the textbox Border Width Property (this applies to a table not a matrix):
=IIF(Fields!SortOrder.Value = 2,"3pt","1pt")
Once I click OK on the expression editor and OK on the Text Box Properties editor, I can go back into the properties, and the Border Width property is again set to the default 1pt.
After playing around with it for a very long time I finally started getting the text box to behave the way I wanted, but the property was still set incorrectly. However the XML of the report showed it being set with the above expression. So SSRS wasn't properly parsing the XML until the run-time of the report (at least that's what I'm assuming). I have been able to set the Bold property on the same text box and all other text boxes within that row. However, all of the properties in the Border section of the TextBox property list seem to reset themselves. I've only had on occurrence of the report actually doing what I wanted (as mentioned above), but the property displayed through SSRS was still incorrect. I guess I need a way to trigger SSRS to reprocess the XML.
Things I have tried:
-closing and reopening report
-closing and reopening solution
-closing and reopening Visual Studio
-restarting computer