I have a friend who like to rent my MT screen to display his pictures for a show, but my concern is, since apple was granted a patent on MT, is it illegal if I sell a MT project or rent out my MT screen?

You can sell everything you’ve made personnaly.
I think, as long as it is your work, you can do what you want.

Understanding copyright and trademark, are very complicate issues, one country is yes, but another country is no.  often, there all legal.  my advice speak with your local laywer for correct answer for your area how individual copyright and tradmark apply in your country.

I wouldnt get to caught up in copyright here.  If you are only rolling out custom made screens as optical multitouch platforms then there is no problem with copyright as the technology is freely disscussed and documented. 

For example the standard shape of an SLR camera cannot have a patent attached as this is the standard form a body a lens.  It is only the inovations that can be patented such as a special auto focus thingy.

In a similar vain the optical multitouch table cannot in itself be patented but the specific technologies used can such as a special screen coating or magical optical setup.

I think if your not going mass production (10,000 units a year) then i wouldnt get to caught up in copyright.

Software however is different you cant repackage freeware and then sell it if you have done nothing to change or alter the code.  Most of the companies who are serious about selling MT soloutions have rolled their own tracking software and offer a system to develop ontop of, however saying that these trackers normally incorporate elements of freeware such as openframeworks libraries etc.

Obviously it can get complicated but if your just selling 10 20 custom setups i cant see the problem.

Disclaimer!

All advice given here is personal opinion and has no relivence in the real world!

ya i think there is no problem, if u sell it with the official leopard version not the hackintosh one or, if ur planning to sell it, using like imac, or macbook pro, then this is legal, as u are only selling their project which you bought, also the mt screen is an output device, which is no problem since u didnt it ur own and got nothing to do with apple, however i would recommend linux, to avoid any trouble.

One thing can be that you make a small contract where it tells that you only sell/buy/rent the computer/multi touch and not the OS!