As long as the sane patient has given proper consent, the arrangement is absolutely ethical. In order to properly measure the effectiveness of psycho-analysis, as it is with any experiment, it is absolutely essential to incorporate a variable measure other results against in order to come up with an accurate result. If David Rosenhan follows proper protocol and gets permission from a sane patient, then there is no issue of ethics in this study.
It is absolutely ethical for Psychologist David Rosenhan to make a deal with a hospital to admit at least one sane patient, in order to test the validity of psycho-analysis, provided the sane patient is fully made aware of the purpose and willingly agrees. How else are doctors able to adequately test theories?
If that patient was treated well and not subject to any experimental treatment, I'd say that's fine. Of course, there's a Catch-22 in play, as admitting them could hurt their sanity, which would negate the purpose of the test in the first place. It doesn't seem innately unethical, though, it just depends how it's conducted.
Rosenhan's deal with the hospital was a clear breach of ethics. The only way that this would be OK is if the sane patient understood what was going on and gave his consent. If not, he was being tricked by people he should be able to trust. Rosenhan should lose his license to practice medicine.