No not yet, is a unsupported toggle in the code for it.. but multiple developers have stated it's broken for now.
People using amd64 still have the access to a larger virtual address space, although I think it's set to around 8GB or so per process.. for now anyway.
As for the NX bit, OpenBSD has always simulated that on most architectures via
W^X.. the introduction of the NX bit on AMD64 just made it simpler.
AFAIK, newer CPU's supporting the NX bit can also make use of it on the i386 platform, I could be wrong though.. they use a different (..slightly more costly) alternative on processors that do not, using a segmentation trick.
A paper by Theo:
http://www.openbsd.org/papers/ven05-deraadt/index.html