Wow - that was fast. Microsoft has now posted the trial versions of OCS for download. The download, which you can get at Microsoft Office Communications Server 2007 Trial Download, is 374MB. You actually download an MSI then then "installs" both the Standard Edition and Enterprise Edition binaries on your system. You can run setup from there to actually install OCS.
Of course, if you are using OCS 2007, you probably also want the updated version of Communicator. The evaluation edition of Microsoft Office Communicator is also available for download here