After having several people poke me in different contexts about why inline cleartext PGP signatures are a bad idea, i got sufficiently tired of repeating myself, and finally documented some of the problems explicitly.
The report includes a demonstration of a content-tampering attack that changes the meaning of a signed inline-PGP message without breaking the signature, which i first worked out on the notmuch mailing list, but hadn't gotten around to demonstrating until recently.
The attack is demonstrated against clearsigned messages, but also works against inline encrypted messages (but is harder to demonstrate since a demonstration would require sharing secret key material for the decryption step).
Please don't generate Inline-PGP messages. And if you must parse and accept them, please consider carefully the risks you expose your users to and think about ways to mitigate the problems.