A "content encrypted" MOBI can be read by any Reader from MobiPocket (no PID required) but can't be read by 3rd party MOBI readers or converters. The standard mobidedrm works on such files with a minor edit, but only if the are in the BOOKMOBI file format.
All such files (even in TEXtREAd format) are circumventable using the Java-based
Secure MobiPocket Decoder, but I don't think this is available on the web any more.