Conficker is one recent example. Exploiting a known vulnerability for which there is a patch, Conficker continues to spread and according to analysis by SRI continues to evolve and demonstrates the creators ability to adapt and enhance the malware. Conficker is sophisticated, to be sure, but it's no where near the cutting edge exploit that a BIOS update or SMM rootkit is. Yet, Conficker has much more potential.
Both attacks are highly specific and require substantial expertise to become effective. In SMM Rootkits: A New Breed of OS Independent Malware, a paper that Rutkowska references, the authors list number of limitations in making a generalizeable SMM rootkit not the least of which is not having OS driver support to utilize installed hardware.
The research is interesting and the attacks are viable, but given the simpler, easier, more wide spread methods of gaining control of a system, hacking the BIOS or SMM probably isn't big threat.