Be mindful that you don't overreact to the 'new' SMM exploit (properly, reported by Loic Duflot, a very accomplished low level hardware researcher, at the recent cansecwest conference). The exploit itself is really a documented 'feature' of the Intel 5100 Memory Controller chipset, and has been a known issue with SMM for quite some time. See the 5100 data sheet:
In order to make cacheable SMM possible, the chipset must accept EWB’s and must absorb IWB data regardless of the condition of the SMMEM# pin. The Intel® 5100 MCH Chipset will not set the error bit EXSMRAMC.E_SMERR in this case. Because of this, care must be used when attempting to cache SMM space. The chipset/platform cannot protect against processors who attempt to illegally access SMM space that is modified in another processor’s cache. Any software that creates such a condition (for example, by corrupting the page table) will jeopardize the protective properties of SMM.
You might ask why it hasn't received more attention until now? Stated simply, such low level tactics are simply unnecessary for a real rootkit to be effective. Remember that you need to be in ring-0 (kernel) before you can even attempt installing into 'ring -1', and being at ring 0 is plenty of privilege for even the most stealthy of rootkits.
There are other reasons that an SMM rootkit is best left to the science fair: to make one that is effective across more than a select hardware platform, you would need to invest boat loads of development dollars in testing. At worst, someone might build an SMM rootkit that works on a well-known and distributed model of laptop and post that for publicity - but the real criminals don't build stuff like this, nor do they have to.
The majority of malware threats today are, in fact, usermode. The bad guys simply don't need to go any lower to get their work done. Remember, the lower you go, the less re-usable code you can leverage. That is, lower means no libraries, no API's. Lower means you write all the device, memory, and hardware logic yourself. It approaches the complexity of device driver development and operating system design. This all translates into expensive and non-ubiquitous. Malware avoids this development cost by simply installing itself like any other program, re-using the existing API's and libraries under windows that already provide network access, memory management, file access, and the like.
The modus operandi of real malware authors is: Write once, use many times. An SMM rootkit is a really neat science experiment and excites technical curiosity, but such an approach is not useful in practice. Let's stay focused on our Day Job, tackling real threats.