BlackbeardSEO writes that when he tried to bulk upload 3,000 keywords to Microsoft adCenter, it returned an error and does not upload the keywords. BlackbeardSEO explained that this only started happening with the new update of the adCenter product this weekend. He also posted this at DigitalPoint Forums, where he confirms that he "called Microsoft about it and they are working to fix it." Microsoft told him that there "is no ETA on when this will be done, but hoepfully soon."
WebmasterWorld explains exactly how many keywords can be uploaded.
You are limited to adding around 500 keywords per time using the normal way.
So the bulk upload feature is not too bulk these days.