Ok I
How about using the PLE version of Maya?
That should only allow them to view it in the PLE version and no where else. I don't think you can connvert PLE files to anything!
Just a thought
--g
I think if you were determined you could write a MEL script to convert the format but it would be a PITA.
H emight beable to use one of those onoine 3d formats to view the model online.. Personally it sounds bit paranoid to me (is your work that good?).. If wouldn't be too hard to determine if they did in fact use your model and you could sue their pants off so I don't really see why they'd take the risk.
No it
Then is showing them a quicktime movie not an option?
Okay, this is really sick...
Put a scriptNode in the scene that has a promptDialog that will run when the scene is opened. You could then conceivably make this work as a password for the file. If the wrong password is entered or the window dismissed, close the scene. This would work for an in-person demo since you could be there to enter the password. If by mail, you would have to give them the password, so now what?
It gets sicker...
If the correct password were entered, a new scriptNode with the same code but a new password would be created and the old scriptNode deleted, and the scene would be saved.
I am not sure if a promptDialog is modal, though, so none of this would work if not. It would be a significant script undertaking jamming all that in the command flag of a scriptNode, but, hey, it might be possible. Probably not worth the effort, since it would still be pretty easy to crack once you knew what was going on.
Still simply having them open a scene that spits out a PASSWORD? prompt when opened might discourage looking more closely and and be pretty flashy to the not so script-savvy 
--JeffD
Theres one problem with this! And its quite big! Anyone knowing maya could just run a script to strip all your script nodedes before they were to run maya!
So the only way to protect your setup is to make some parts of it hidden in a api node (thats one reson to use api)
As for ple files i think (some insider info seeped to me but i have no idea how close this is to the truth) they are just crypted normal maya files (either ascii or binary). And i know AW can do conversions between...
well one could decrypt them given a century or so to uncrypt the key... but thet offcourse would be against any laws around.
But one can use mel to export between ple and maya... its just not very handy...
edity edit:
joust one second tought... I think its very hard to prove someone swiping your setup. I mean, what is to say that your the only one with that idea! Youd need to prove that!
(also they can do reverse eingeneering totaly lawfully just make a playblast and then tell someone else to do just that, unless you patent the idea... just to add some paranoia)
besides... even a ple setup would be useless since any good setop artist would know a good setup when he sees one. and be able to most of the rigging without even digging in the hypershade (give people some credit) . And by digging would spot all your gems and flaws. (its easier to sometimes disect other peoples setups)
Yeah, Joojaa, I think almost any of the regulars here on the MEL forum could get around something like this. I also was thinking that importing the scene would be a real easy way around it. I don't think anything effective file protection is possible with MEL alone, but it was certainly fun problem to brainstorm on.
--JeffD
Yeah but you could hide the things inside some isntructions on a code and use the password as a sortoff decryption key maybe... offcourse no protection for save so maybe just ecrypt the file!