What versoion of EPiServer are you compiling towrds?
In all recent versions, there is a property "ID" on the XFormData class that returns the id of the individual formpost.
I have 4.62, and when i try with formData.ID, I get "Episerver.XForms.XFormData does not contain a definition for 'ID'. "
Try with "formData.Id" (non-capitalized 'd' in 'Id'), it really should be there...
Oh! There it was! Thanks! Visual Studio didnt help me at all with that one. When I pressed dot (.) and looked at the list of methods in my formData inctance it wasn't there.