IOMMU, not regular (CPU) MMU. The FAQ _does_ address this, but it's under "What about DMA?". In short: drivers have to be trusted for now, except that there's experimental support for x86 VT-d (which is a type of IOMMU).
This is all essential trust anyways. The leaps and bounds we've achieved through hardware engineering have the burden that they aren't credible for security. You can use IOMMU, but perhaps I won't. Integrated co-development of hardware and software is ideal, but generally there is an adversarial relationship, and we must reflect that in the software. Trust and security are not yes/no questions. We have to keep pushing boundaries. seL4 is a good start; let's make more from it.