Actually, it varies from state to state (Just like doctor-patient) and, in fact, you could be compelled as an attorney to testify against a client in plenty of cases in plenty of states.
In a lot of states, for example, you can compel an attorney to testify about future or ongoing crimes, but not completed crimes someone is seeking legal advice for.
As an example, this is covered in california evidence code as section 956:
"956. There is no privilege under this article if the services of the lawyer were sought or obtained to enable or aid anyone to commit or plan to commit a crime or a fraud."
In a lot of states, for example, you can compel an attorney to testify about future or ongoing crimes, but not completed crimes someone is seeking legal advice for.
As an example, this is covered in california evidence code as section 956:
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=evid&g...
"956. There is no privilege under this article if the services of the lawyer were sought or obtained to enable or aid anyone to commit or plan to commit a crime or a fraud."