Most things work on AMD64. In SmartOS, KVM was the major feature that didn't work on AMD64, but as they said in the faq, the community has been working on that. I can't currently find the testing ISO builds with that support, but here is at least one repo with KVM on AMD support: https://github.com/jclulow/illumos-kvm
Joyent's original work is worse than just Intel only, it requires features found in Core iX and relate Xeon CPUs. This means it doesn't work on Xeon 5xxx chips that are getting affordable and it doesn't work on C2D chips that support VT-x. It sounds like AMD support is further along than supporting older Intel chips.
That's my repo! The "pre-ept" branch contains experimental support for both AMD CPUs with AMD-V support, and Intel CPUs lacking the EPT feature (Joyent targeted EPT support with their initial port.)
I have a hacked up SmartOS ISO from a month ago with a build of the "pre-ept" KVM module replacing the stock module. If you'd like to test it out, with the disclaimer that it might not work for you, it's here: http://alpha.sysmgr.org/smartos-20120223-jmc2.iso
If it works for you I'd love to hear about it. If it doesn't work and you can provide crash dumps or even remote console access to the machine for me to debug the problem then I'll try and help out. My goal is to get the new support tested and working on as many different bits of hardware as possible and then get it integrated into Joyent's repo once cleaned up and proven.
A lot of us hang out in #illumos on freenode (IRC) if you'd like to chat -- I'm LeftWing.
EPT (extended page tables) which are required by the Joyent work made it that much simpler to port the KVM stuff over. EPT simply a lot of the work required and reduce overhead for virtual machines.
Given the size of the community, it's likely that Illumos will support less and less hardware over time. You may want to stick to whatever's on the Nexenta compatibility list.
Relevant to this discussion : Joyent releasing SmartOS http://news.ycombinator.com/item?id=2887170
AMD support from SmartOS wiki: http://wiki.smartos.org/display/DOC/SmartOS+Technical+FAQs#S...