Glad you feel like you've figured it out, Meg's Mom!
Since I go to the archives so much to research various issues (thanks MDC!) even though it may not apply here I just want to say for the record that with dd, it was a boundary thing. Aside from very early on (like her first week), I'm like Breathe, and only nurse when dd asks. ("Milk" was one of her first signs, at like 6 month old, so communication wasn't an issue.) Dd did the biting thing just when she was doing other boundary-testing behaviors -- can I grab the lamp? No. Can I throw the heavy rattle? No. Can I put the pillow on the floor? Yes. Can I...? All of these were accompanied by little grin and watching me closely for my reaction -- which was exactly her expression when she bit.
Of course, it could all be wrapped up in one thing -- Meg chooses to do the boundary-testing when she has needs other than nursing, rather than when she really needs to nurse.