Chris,
You will need to use the current() function. The problem is that you are calling getDOM() and it is returning only the top row of that repeating section, your XPath will look something like this, but if this doesn't work, try playing around with other relative paths such as current()/../../ etc.
Textbox 1 - xdXDocument:GetDOM("SMARToolData2")/dfs:myFields/dfs:dataFields/d:tblSMARToolData2/@Project_TaskID[../@Common_Title = current()/my:ProjectCommonTitleCI]
Also, check out this HowTo:
http://www.infopathdev.com/howto/tutorials/default.aspx?i=9111ef580f95479890fc18fb2c5bac85
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge