Yes, I just had the same issue. I have a "staging" site that is simply a way to test the app in the production environment but with different datastore. In order to test the blobstore related functionality I need to enable billing but since now enabling billing will soon costs $9/month my
staging environment is no longer free. I'm taking about 100s requests per month. Not bad ROI for google. Down the drain the idea that app engine was design with the little guy in mind.
It's pretty disappointing that this scenario hasn't been considered.