The FreeBSD 8.1 schedule can be found at FreeBSD.org. Before making the July release, two release candidates are expected in June. There is also a Wiki page that's tracking what needs to be done to this point release.
The FreeBSD 8.1 beta release can be installed by using either the provided installation images or using FreeBSD's built-in package update mechanism.
The release announcement for FreeBSD 8.1 Beta can be read on the project's mailing list.