I have been shooting my new 85L on my old 20D and I have discovered that what I see on the viewfinder doesn't exactly match what is captured by the sensor. This is true especially at wide apertures. For shoots between f1.2-1.8 my viewfinder shows me a lot more depth of field than what is captured by the sensor. For instance when I shoot portraits the background seems somewhat in focus in the viewfinder but on the photo I get the creamy bokeh of the 85L. Of course I like the photo but is it possible to see the F1.2 effect in the viewfinder on a crop? I am thinking of upgrading to a 5D soon but was wondering if this was another disadvantage of a the crop factor.

The smaller the image, the more it's ADOF (Apparent/Acceptable Depth of Field) is. Doesn't have anything to do with crop factor..

DOF is actually a function of print size - which controls the size of the circles of confusion. That is why smaller prints (viewfinder) look sharper, front to back than do larger prints.

A good primer on DOF...
http://www.bobatkins.com/photography/technical/digitaldof.html

If you need more, search his site for DOF - a lot of material there.

I think it might be related to the focusing screen as well. I use an EE-s screen on the 5D and it shows depth of field better than the stock screen, but the problem is it is darker with slower (f/4) lenses. It's something about the microprisms.

The 40D has a replaceable screen I believe, though I imagine the effect is still easier to see in the larger 5D viewfinder due to the reasons mentioned by others above.