I went out for a photo shoot today with my 7D, Tamron 17-50mm f/2.8 non-VC, and my Canon 100mm macro. I took about 30 pictures with each lens. With the same ISO & aperture settings, shutter speed 1/500 or more, and focus point selected on the eyes, every single 100mm picture was "hazy". The 17-50mm was consistently OK.
After I came home I tried to replicate this problem indoors and I couldn't. I checked for focus issues, and there is no back focus/front focus issue using a chart.
It was sunny outside and I was using a flash. The problem occurred when the subject was back-lit, front-lit, side-lit, and in the shade...like I said, every single picture. I never noticed this with my XSi and today was my first time outside with the 7D.
Examples below, 100mm first and then 17-50mm @ 50mm. SOOC, no sharpening. ISO 100, f/4.0, 1/500