Anonymous - 2010-05-05

When trying to post a sample product feed, I got the following:

2010-05-05 13:25:30,951  ERROR com.amazon.merchants.transport.logging.AuditLogger  Could not post document products.xml Message: The Amazon server has reported an error. The text of this error is: (400)Bad Request. Root cause: The Amazon server has reported an error. The text of this error is: (400)Bad Request.

Does this mean that my xml feed is bad, or what?  The error message doesn't provide much detail, so how might I further narrow down the problem?