Hi Patrick,
If you have a browser-enabled form and go to Form Options > Digital Signatures, you'll see that the option to sign the entire form is not supported by Forms Services, but signing parts of the form is.
As a side note, you'll also see that the option to prompt the user to sign the form before submission, if not signed, is under the 'sign entire form' option, which is disabled. That is why I asked how to do this programmatically in my previous post.
Anyway, what I did is I added the root element of the schema as the part to be signed (I fail to see how this is not the equivalent of signing the entire form?). I then wrapped most of the controls with a section control which can be digitally signed.
The "Click here to sign this section" link gets rendered in the browser once published. It seems that Forms Services supports this through the use of an ActiveX control which the user must install. Out of curiosity, I checked whether this would work in Firefox, but got an alert saying that this browser is not supported. Back to IE, as soon as I saw the pre-release disclaimer for the ActiveX control, I decided not to install it since we have MOSS 2007 RTM on another server and this test server has B2TR installed. I'd rather install the final version of the ActiveX control and thus have not tested whether signing parts of the form actually works in the browser yet, but it should.
Thanks for the information on the Sign method, I'll do some research tomorrow and see if I can use it to achieve my goal.