Adobe has posted a release candidate of Lightroom 2.1 for those Lightroom 2.0 owners who wish to try it. The update has no new functionality (except support for some new cameras), but does include a host of bug fixes and speedup improvements (all listed on the page I linked to). If you're being bit by one of the bugs, you'll probably want to upgrade.
The “release candidate” status means that Adobe believes it's ready for use, but it's not an update that's pushed to everyone automatically; you have to download it manually. After the Lightroom 1.4 booboos, Adobe is stepping more prudently with this release.
Eventually, Adobe will release 2.1 as a normal, full release. When it does, it may well turn out to be exactly the same as the release candidate. Or, further refinements may be made between now and then. We'll see.