There appears to be a funny bug in Preview.app 5.0.3 (504.1):

I took a few PDF pages of normal black text, used Preview's yellow highlight functionality, and printed it on a B&W HP Laserjet.

The pages that don't contain any highlights look normal. But the pages that do are printed in a strange 'jittery' low-quality -- all of it, not just the words that are highlighted.

I've seen this happen with a variety of different PDFs that I've highlighted using Preview.