Short answer yes. Long answer, I don't think anyone has tried a 56k modems with the 1.14/1.15 revamped modem handling yet. They might work now for all I know. It would require significant testing before committing to a real account though.
A more interesting question is, now that we have survived Puerto Rico and Fresno, are the Digi AccelPort/MODEM boxes on the official "supported" list. I suspect the short answer to that is "yes", although the people having to do said support should make the call.