How to display the Form version on the InfoPath 2007 Form

Just insert an Expression Box control into the Infopath Form using the following XPath expression:

substring-before( substring-after( /processing-instruction()[local-name(.) = "mso-infoPathSolution"], 'solutionVersion="'), '"')

Source: http://www.nivot.org/2008/09/30/WhyVSTO30VisualStudio2008SharePointWindowsWorkflowAndInfoPathMightGiveYouAHernia.aspx