I have a 68 Camaro and I would like to change the body mount bushings. The engine and transmission is in the car. Can these bushing be changed without removing the motor/trans and completely removing the sub frame?

I haven't done it yet, but I have heard the answer is yes. You are supposed to do only one side at a time to aviod changing the body/frame alignment.

good question,about to do a complete rebuild on my front-end body bushings,ball joints,tie-rods,and so on...so let me know how easy it was...

You could replace them either way. I have replaced mounts on three different cars and on each car had to rebuild the body mount pads on each one.
My suggestion would be to inspect the frame and determine what method to use. If you need to rebuild the subframe it should be removed frome the body. In one rebuild I left the engine and trans mounted to the subframe and rolled it out from under the body.

You can definitely do this job without dropping the subframe. I replaced my original GM rubber subframe-to-body bushings with Energy suspension polyurethane units in my driveway with jackstands and a cheap floor jack. The key is to do one side at a time like someone else above said. Do the radiator support bushings first and work your way back on each side. That's what I did and it is no big deal. You will probably have to loosen more than one mount at a time to try to get some clearance inbetween the mounts. Be sure to use plenty of WD-40 on the bolts if you aren't replacing the bolts with new. Also make sure you have large enough sockets and tools, because those are some really big bolts. Replacing the rubber units with polyurethane really improved the feel of the car without introducing a lot of vibration. One other thing: GO GET A PROFESSIONAL ALIGNMENT AFTER YOU DO THIS JOB!!! Good luck!