You can download it here:
http://www.ubuntu.com/testing/dapperbetaUnder the downloads get the Desktop install for an easy graphical live CD with espresso install and also a persistant option for USB keys/floppy settings, or get the text mode install ISO for advanced/server installation.
OK, a couple of things, Ubuntu Dapper Drake was scheduled to be released this month, as in version 6.04, April, 2006, however, they decided to delay the final release till June, hence the version change. All Ubuntu releases are called "year.month", and are usually scheduled to be released every 6 months. So as a compromise, they released this beta version instead. Mark Shuttleworth has stated that they want this release to be the best yet, and they changed the schedule accordingly. Another reason is that all Ubuntu Releases before Dapper were supported for security updates and bugfixes for about 18 months, Dapper is going to be supported for 3 years for the desktop and 5 years for the server/enterprise. They warn that this beta version isn't a "stable" release, but let me just say, being a Dapper tester myself, that even the Alpha releases were extremely stable, and after a clean install of this beta version, I have not encountered any problems, yet. Again, you have been warned, BACKUP your data, or better yet, keep it on a separate /home partition, at the very least.
Now for even more confusion, of sorts, they are planning a new version codenamed "Edgy Eft" or Dapper+1, basically its supposed to be a bleeding edge release integrating experimental technologies like XGL/Compiz and/or Aiglx in addition to others. An alternative for those who like living on the bleeding edge of technology. No real goals have been laid out for this version, but Mark Shuttleworth basically said its for the developers to decide and says it will suffer from stability problems and such.