I just noticed this issue, which never used to occur, but I'm not sure what has changed to make it suddenly happen.
My WP E-Commerce checkout page correctly calculates North Carolina state tax for customers with NC shipping addresses, with the appropriate total in addition to shipping, but once you click through to Paypal, the tax calculations have disappeared. Logging into Paypal and completing the transaction does not affect anything.
Does anyone have any idea why this might be happening, or how to fix it? I've tried adjusting the Tax settings under Store Settings, but to no avail.