As the need for Two factor authentication is a requirement for PCI-DSS (Payment Card Industry standard) and SSH Key with password is not always deemed to be an acceptable form of Two factor authorisation there is now a surge in different forms of two factor auth, all with their own pros and cons.
(Disclaimer – I’m a Rackspace Employee, the postings on this site are my own, may be bias, and don’t necessarily represent Rackspace’s positions, strategies or opinions.