It could be a problem with your DLL reference. There are two versions of the InfoPath 2007 OM, one for browser forms, and one for client forms. It sounds like you're referencing the browser version.
My MSTA shows the browser version existing at
C:\Program Files\Microsoft Office\Office12\InfoPathOM\Microsoft.Office.Infopath.dll
and the client version at
C:\Windows\assembly\GAC_MSIL\Microsoft.Office.InfoPath\12.0.0.0__71e9bce111e9429c\Microsoft.Office.InfoPath.dll