Well, I gotta ask, if you have a modal form that's so long, shouldn't it be made into its own page?
I mean, the whole point of modal dialogues is to tell the user something he needs to know (which are usually disregarded and are annoying) or to get some information from the user that is necessary before proceeding.
You say your form is for gathering user input. If it's something the user must enter before proceeding (as in a part of a checkout flow or something like that), then I would say it's probably best to dedicate an entire page to the flow.
If it's something that's more of a "log in here before proceeding with what you're doing" kind of thing, again, I think it would make more sense for it to be its own page that brings you back to the page you were on before you entered it once you're done filling out the form. That's how the Stack Overflow human-verification page works.
If it's something annoying like "give us your feedback about the site", then it shouldn't be modal at all but rather an easily-dismissed (dare I say it?) pop-up window.
Modal dialogues really should be kept as brief as possible. If brevity is impossible, and the dialogue really must be modal, then I think it would make more sense to create a flow of pages that must be filled in before the next one can be accessed. Like a checkout: you need to add products to a cart before adding shipping information, and you need shipping information before you can determine the cost of the shipment. That kind of thing.
But without knowing the exact nature of your modal dialogue I can't tell you exactly which way would be best.
EDIT: Aha! Your clients felt this was time consuming, eh? This is the type of situation where you should do a very quick and dirty live usability test to see which way is actually better. Grab some people from down the hall, show some of them the modal way (with scrolling) of doing it and show others the old (non-modal) way of doing it and see what they have to say.
(Ideally you are recording the session and the screen, and you make sure to not let your own personal preferences show through. Just ask them to use the system while you watch to see how well they perform the task. Use the recording to time both methods to see if one way really is faster than the other.)
You shouldn't ever make a usability decision that goes against the norm (the norm in this case being "large forms merit their own pages") without making sure that it actually is more usable the abnormal way. When it comes to usability, the norm is usually the norm because it's usable (but not always, which is why you must test). If the client fights back, you'll at least have evidence that they're going against hard empirical evidence that what they want is silly.
Ultimately, though, the clients are the ones paying the bills. If you can't get them to see reason then you'll have to make the most of what they tell you. If the form must be in a modal dialogue, then you can at least try to hide non-essential fields under the fold (if there are non-essential fields) so that the majority of users will never have to scroll.
Make sure the buttons to submit the form (or whatever it is that you need to do with the form) are visible no matter where the user has scrolled. A really bad idea would be to put all the required fields at the top and then force the user to scroll down anyway to hit the submit button. That's just rude.