The SharePoint team originally primarily targetted information workers in providing a workflow development environment for their SharePoint Designer product when they released Windows SharePoint Services v3. For developers who prefer Visual Studio for creating workflows there is now the Workflow Starter Kit as was announced on Randall Isenhour's blog.
I think that software developers will be just as much workflow creators for SharePoint as information workers, if not more. Workflow in SharePoint is very powerful and it's natural for software developers to want to take advantage of this. But the tools are like a trickle turning to a stream turning to a river. We don't have the river yet, but we also haven't had a release of Visual Studio since the Windows SharePoint Services v3 RTM. I can't wait until that comes around, but until then you want to get one of these packages for Visual Studio 2005.
There are two packages available for the Workflow Starter Kit for SharePoint and VIsual Studio:
MOSS SDK and ECM Starter Kit – We combined the ECM Starter Kit with the MOSS SDK documentation and samples, and we also included the WSS and Office Forms Server SDK documentation so you only have to download and install one package.
Windows SharePoint Services Developer Resources – If you are only interested in WSS platform development, we combined the WSS SDK documentation with the Workflow Starter Kit.