I have an old Nikon 50mm F 1.8 lens that I use on my Canon Digital Rebel XT. Whenever I use it, I have to shoot manually. I understand that the body can't change the aperture, but why can't it expose with any aperture?
When you shoot with a manual lens, with no lens attached or a lens that can't communicate with the camera (tested on the 550D/X2i, I believe this applies to all Canon DSLRs) you have 2 options:
Shoot in M mode - you need to set everything yourself but the exposure indicators works and is about as accurate as it is with a Canon lens.
Shoot in Av mode - the display will show the aperture is 00 and won't let you change it but it will choose the correct shutter speed for you.
Obviously you need some way to set the aperture manually and you need to set it before metering with the camera.
All the other modes won't work because the camera has to be able to set the aperture to use them.
Note: I didn't test this with auto-ISO
It really depends on how old the Nikon lenses are. As others have said the key is that the lens needs to tell the body what aperture it is set to. And the body has to be able to tell the lens to stop down.
I've got a bunch of Nikkor lenses that are for my Nikon F/Ftn, which uses an external prong to couple the len's aperture to the body. Look at any old Nikon lens, and you will see the prong. http://www.momentcorp.com/temporary%20files%20for%20website/nikon_lens_versions/nikon_pre-ai.jpg
In the early 1970s, Nikon came out with the "AI" connection, which replaced the big prong with a small notch in the aperture ring. The Nikon F2 and later bodies uses this notch rather than the prong, but the effect was the same.
Old Nikkor lenses have a small lever sticking out of the back of the lens that connects to the body so that the lens can be stopped down when you shoot.
All this is done mechanically, no electronics, or computers are involved.
Because there are no CPU connections between the lens and the camera, there is no way for the camera to know what the aperture is set to, so it can't calculate the exposure.
I use old manual lenses, too, and it does work fine with auto-exposure in Av mode (sets ISO and shutter correctly) and M mode (the exposure indicator works) and I also get the focus confirm light (chip on the adaptor). The benefit of the chip is that I do get exif information in the files. The new EMF chips can even be programmed to show the lens data e.g. 135mm F/3.5 (the normal chips always show 50mm 1.4) and has AF micro-adjustments.