I created a feature on site A and then enabled it on site B, which worked as expected. But when I disabled the feature on site B, the content types created by the feature were not deleted. I've confirmed that in the D6 version, content types created by a feature are subsequently deleted when the feature is disabled (even if you have created content of that type), so I assume this is how it should work in D7.
Current workaround 25 July 2012:
Add a delete at the end of the URL.