MongoDB 1.8.0-rc0 is now available, you can download from here and find the release notes here.
The main changes over the current 1.6 production release relate to:
durability, a much talked about topic (!), with support now for write-ahead journaling (docs)
support for more Map/Reduce functionality (note there are some breaking changes in this area)
The full info on the changes is in the Release Notes document.