You can get a literal eqivalent to PEEK and POKE by mmap()ing /dev/mem. You can map the MMIO regions of a PCI device to write a userspace driver for it, for example. This is how I prodded the tl880 chip before I had a kernel module ready back when I was working on that driver.
You don't get interrupt handlers, for one thing (though I think there are projects that do allow complete userspace drivers), and as binarycrusader mentioned, it's incredibly dangerous to give any process full access to the entire physical memory space. A single vulnerability in that process gives the attacker the ability to write to all of system memory. This can be mitigated somewhat by mmap()ing only the smallest possible region of interest, then dropping root privileges.
Yep, although most linux dists now have "/dev/mem protection" meaning that on stock distro kernels (Ubuntu, CentOS, etc), access to actual "physical memory" is disabled.
It's a giant, gaping security hole that most operating system architects are desperately trying to get rid of -- at least, that's the way I view it.
Any software that uses it should be exposed to extra scrutiny and must judiciously leverage whatever privilege mechanisms an OS provides to mitigate damage.
There should be a better way to accomplish what xorg needs on *NIX systems.
