Multi-platform two-factor terminal services?

[Mike Osterman <ostermmg () WHITMAN EDU>]

I'm currently investigating adding two-factor authentication for
terminal services. It appears that there are products ready to go for
Windows (http://www.duosecurity.com/docs/rdp), but I'm looking for
something that would work for both Windows and Mac clients. The Mac
Remote Desktop Client that comes with Office 2011 does not appear to
support this capability.

The terminal environment need not be Microsoft Terminal Services if it
can support Mac clients as well.

Does anyone know of such a product? I'm willing to hear from vendors
_directly_ with the understanding that I am simply looking for ballpark
pricing.

Thank you,

Mike Osterman
IT Security Officer/
Deputy Director, Enterprise Technology
Whitman College