Strange that none noted that identity based encryption (IBE for the acquainted ones)solves this problem quite easily (more on
http://www.voltage.com/technology/ibe.htm). Boneh and Franklin scheme was the first proposed one, but nowadays this is not only on crypto papers, but they are even RFCS for such schemes:
http://www.rfc-editor.org/rfc/rfc5409.txt. There are even some non-commercial implementations around:
http://crypto.stanford.edu/ibe/.
Of course, not using such full blown solutions will mean that posterous' heuristics techniques will be susceptible to all sorts of attacks, such as man-in-the-middle, relay attacks and so forth.
On the other hand, looking for solutions that are resilient to more sophisticated attacks, mostly considering IBE schemes, is quite convoluted (it involves provable security models, such as http://www.google.com/#hl=en&q=provable+security+signatu... ). There are even variations on IBE, such as certificateless, which require you to trust even less people.
This is of course, assuming you are not willing to inconvenience users by making them reply a email you send them after they tried to poste. Such email would contain a custom made url (the secret) that would enable the post to actually be posted. On the other hand, this solution feels more inconvenient than using OAuth methods.
Nonetheless, not all users care about security/privacy (those that do, will always have the usual login scheme). If you chose to go other way, good luck to you. After all, people still use MD5 for security applications nowadays.