Why are my exterior real-estate photos soft on a Canon 50D when the same lens looked fine on my Rebel XTi?

Rodger Cicala over at LensRentals.com would suggest that in fact your lens is not soft, or at least you only have a 3-7% chance of that being the case. You said that you used this lens previously on a Rebel XTi body with good results, and now with a new body the 50D you receive poor results. Rodger contends that this is due to slightly different characteristics that each body can have.

A lens may be fine on one camera and not another. A camera may do fine with one lens and not another.

Another great quote to note

Until recently, wide angle lenses tended to be soft in the edges and corners and people just accepted it. Recent lenses are so much better that differences from one side to the other became noticeable (the difference between sharp and soft is more obvious than the difference between soft and softer, apparently).

It is possible that you have a miscalibrated lens or body, or even both, but a respected(in my opinion) professional with a great deal of experience believes that it is due to the differences between bodies and lenses. A suggested solution would be to send the lens and body combination into Canon and the can properly calibrate the two together.

More could be determined if you could provide sample images. If you find this reply to be the best that you receive, please mark this as the accepted answer.