According to a Microsoft announcement the Express Edition of SQL Server 2008 won't be available until the end of August.
This however turns out not to be entirely true. Itching to try out the new version, I ended up downloading the evaluation version. It turns out that during the installation you get to choose between installing the evaluation and the express versions.
Unfortunately I can't get the management studio to install though (would have loved to try out the new query visualizer). I don't even see it in the install options, and executing the install directly from the DVD throws an error. At least my tool seems to continue to work with the RTM version.