Unless users of oldIE versions contribute more revenue than it takes to support them (which is obviously app/site-specific), screw them. It's simple economics.
More the reason I think jQuery could be shooting itself in the foot. Its ubiquity is based on the fact that if you're doing client side javascript, jQuery is the best option. Coupled with what I started this comment with - its possible that jQuery isn't the best option for everyone anymore.
I personally am not a huge fan of frameworks. Too often I see people solving problems by adding frameworks. Maybe I'm just a crusty guy who doesn't use a lot of tools.