Are you using 2.4? That was designed to not cause an error if the decorated class didn't have a SubsiteID. There's a [url=http://open.silverstripe.org/changeset/100932/modules/sapphire/branches/2.4/core/model/SiteTree.php]code change in the core[/url] that goes along with this. If you're using subsites/trunk for developing a site, you might want to wait until 2.4 is released, as it's probably not entirely stable right now.
Note that without this change, the CMS would sometimes crash because a SQL error (no SubsiteID).
Unfortunately there's no tagged release for subsites that works with 2.4 yet, as we are still testing 2.4 and getting it stable.
If you're brave, you can checkout http://svn.silverstripe.com/open/phpinstaller/branches/2.4 which is the development branch for where 2.4 releases are derived. We're currently working on this branch to get the next release of 2.4 out.
Alternatively you can patch your beta installation with this changeset: http://open.silverstripe.org/changeset/100932