they probably bid the project from the DD docs and now want to see how much more money the Owner owes them for all of the stuff you've added

That was what came to mind first for me too...and is obviously something I want to avoid. I just don't see the benefit to the project as a whole if we give them an outdated model.

Ask them why they want the comparison, and have an opened mind. have the owner participate in the meeting.

Our estimators in Pre-con will load a new model and an old model in to Innovaya. It will graphically highlight EVERYTHING that has been added, deleted, or changed. It just tells them where they need to focus their efforts, instead of basically "re-pricing" the entire project, which is what you are telling them to do if you ONLY give them the new model.

Less work for them = better project?

I would be very hesitant to give a contractor Revit models. The legal issues around the use of and liability associated with BIM are far from settled. I recommend that you give them drawing sheets for comparison. As stated above, it is common practice to issue clouded revisions showing the extent of any changes. If they insist on a model I would give them a 3D DWF.

Or do that. Honestly, i hope more firms keep refusing to share data. It makes Integrated Companies look that much better by comparison, because we play nice with ourselves.

The legal issues are virtually gone, because no firm worth their weight in paper is going to hand over anything (PDF included) without a file transfer indemnification saying the documents override anything they see in the model, which you probably made them sign as part of your front end anyway.

Sometimes it REALLY IS just about someone trying to do their job more efficiently.

I have had many contractors use my model that was either given to them from the Architect or pulled of the projects FTP site.
This last contractor made his shop drawings from my model...copyclip. I do the work, he collects the fee.
As long as you have a disclaimer or something in your contract covering you, I wouldn't be too worried.

I agree with this. Send the model along with a solid disclaimer and you should not really have liability issues (assuming a standard project delivery arrengement here). Given that, I think much of this new process revolves with managing expectations about how and what the model is, can be and should be used for.
If your model is purpose-built, by contract, for the GC to estimate off of, then I suspect that you would not be asking this question. As it stands, you are likely building the model primarily for generating CDs, the paper copies of which are the legal contract documents. Explaining this with both the GC and Owner present so that they can ask questions and have them answered will build good will, and show everyone that you are interested in sharing information so that the project has a better chance of success.
If you feel that you should be compensated for providing the model you can explain why. Obviously, there may be other things going on that might affect your final decision, but on its face, does not sharing the model really help get a better project built?

I also don't see the problem with sharing (if provided with a solid disclaimer). It just helps the GC. I do it all the time...
There are occasions where I get paid to share it. There are occasions where I get paid by the GC to provide multiple construction possibilities. It all depends on the situation. My advice would be to keep an open mind and try to expand your business... I usually tell the GC that I can hand it over as built, or he gives me the specs of what info he needs and I'll set the model up to best serve his needs before handing it over (against a reasonable fee that is). You'd be surprised...

I am not sure that you can rely on a simple disclaimer to fully protect you from potential liability involved with releasing BIM models to contractors. At this time, there simply isn't any case law that clearly defines the legal issues surrounding BIM. Just because a disclaimer was adequate for CAD files does not mean that the courts will come to the same findings with respect to BIM models. The legal similarity of CAD to BIM is not apples to apples. Releasing BIM models into the wild puts you in uncharted legal territory. Do as you please - we need lawsuits to go to trial so that the rest of us will have new case law and a new set of rules to go by. The very first BIM lawsuit was just settled last month (May, 2011), there will undoubtedly be many many more to follow.

A few quotes from attorneys and other industry pundits regarding BIM.

"Like other innovations before it, BIM is treading on new ground, figuratively and especially legally. Although an established and broadly-accepted design and construction production and coordination tool, BIM’s legal history is almost nil; in effect it is a legal Wild West until some general guidelines and benchmarks are set. As owners sift through the fact surrounding this case and what it all means philosophically for their projects, consider what this means for even more nascent and less-accepted industry inventions like integrated project delivery (IPD). The risk averse out there may want to wait several years for IPD’s legal watershed moment before diving in with both feet. "

"It is our opinion that BIM will not only change the way buildings are designed and constructed, but also the way the construction industry operates. Unfortunately, as with many legal issues, published case law will lag years behind project experiences in terms of understanding liability with regard to the use of BIM. For that reason, all parties involved must be mindful of potentially new risks while embracing a technology that may ultimately reduce litigation related to construction projects."