In order to test whether it's the equipment, you must use a tripod. Additionally, do a series of test with different apertures. The lens may not be as sharp at f2.8 as it would be stopped down. If shooting wide open all the time isn't an option, you may want to look into a Nikon 28-70mm or 24-70mm zoom. They do cost more than the Sigma, but there is a difference in quality that comes with the additional investment. Both are excellent wide open at f2.8.

Quote:
I'm an SLR newbie shooting with a D80 and a Sigma 28-70mm f/2.8 lens. My shots are not always crystal clear. Sometimes they are but rarely. At 100% full rez. (10.2mp), my images are about 95% sharp and slightly "soft". They have a faintly blurred halo especially around the highlights. I'm usually shooting at 100 ISO, 2.8 - 3.2 Aperture Priority.

Also, I use an SB600 and I usually bounce it straight up with an index card rubber banded and the diffuser in place.

What might I be doing wrong?

One possibility it the lens itself: Brand-new Blur

Autofocus and/or manual accuracy is also not a given: Focus Accuracy

And don't stop down too far: Diffraction