If you want to start developing on Speech Server 2007, you should probably start off using the Speech Server 2007 Developer Edition – a free, yet fully functional version of the retail product. You can download it here. Please be sure to follow the steps in the Installation Notes exactly, otherwise the installation may fail.
The Developer Edition is only free for developing your application. You'll still need to purchase Office Communications Server 2007 licenses once you're ready to deploy your application in a non-test deployment. You can read more about Speech Server Licensing here.
Speech Server 2007 works with Visual Studio 2005 not Visual Studio 2008. After installing Speech Server 2007, you'll also need to install a patch for a bug that occurs when you have .NET 3.0 SP1 or Vista SP1. You can download that patch here.
For support, you can use the Speech Server 2007 Community Support forums here. For bugs, please file the bug in the ECS-UC-Dev queue, or under OCS 2007.
Documentation is found here. White papers can be found here under the technical articles section. For example, here is a white paper on capacity planning.
The Speech Server (2007) MOM pack can be found here.
Additional web resources: