I'm just starting out looking at SharePoint workflows - all the information I've seen so far assumes a document-centric workflow. Can you do generic workflows (something like BPM or BPEL)?
Yes you can do whatever kind of workflow you want. There are three different levels of functionality however.
Those actions that are configurable from within Sharepoint. These are simple workflows like notifications and simple approval processes.
Actions available in SharePoint Designer. There are a lot more actions available here. You can use them to build complicated workflows, assuming that the provided options are sufficient for your specific needs.
Actions created via Visual Studio. Using Visual Studio you can create custom actions that do whatever you need them to do.
Hope this helps.
Document centered workflows are the norm. You can associate a workflow with a content-type as well. A common scenario is to create a custom content-type that has a field that contains a link to a document from a doc library. That way you can customize the meta-data used with a document without expecting the document itself to house the fields or expecting the document authors to fill in a bunch of meta-data.