There is a clearly defined (and open) standard for supporting MFA in the TOTP standard.

http://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm
http://tools.ietf.org/html/rfc6238

The number of external integrations is limited to Google, and while it would be good to also integrate into Amazon, etc, for smallish (and for users NOT in a Google Apps domain, external contractors as the documentation even suggests, stronger authentication should be supported.
Having integrated the standard already, I know how trivial it is, so if we could get this done, it would REALLY allow organizations who need stronger authentication look at the product.