Tricks I have read for optimizing the load time of large forms are not helping. Linearization causes the first page to render quickly, but you can't interact with the fields until the whole form finishes loading -- no help there. Is there a way to break the form into pages (without creating entirely separate forms) so the user can fill out a page, hit a Next Page button, fill out that page, etc.? Understood that this is an old school idea, but until Reader can download a 1+ MB form in less time than it takes an average user to get ticked off, old school might do the trick.
Alternatively, is there a way to construct a form so you can start interacting with it without having to wait for it all to load? This question comes from the (uninformed) assumption that maybe there are forward references that can't be satisfied until all the bits have come over the wire. If that's right, can a multipage form be architected so as to avoid this problem?
No that technology does not exist yet. There are form level events that need to have the entire document there before they can fire. Also you would have to keep track of where you are so that would mean some sort of session information for each user.