I’m a real fan of SharePoint 2010 composites, so since the beta version came out, I have been playing a lot with most of the features of BCS, SharePoint Designer 2010 Reusable workflows, Access Services and others.
While I was preparing a demo about BCS and External Lists for Egypt SharePoint User group, I noticed that you cannot associate workflows to SharePoint external lists. I double checked that with some Microsoft guys and I reached to a conclusion that SharePoint 2010 External lists doesn’t support the automatic triggering of Workflows however using custom actions ( which could by easily created now using SharePoint Designer 2010) can do the trick, in other words, clicking a button on the ribbon on an external list and triggering a workflow could be a workaround.
What if you in a real need to automatically trigger workflows for external data ? you can create a SharePoint list with external data columns and treat your list as any native SharePoint list. However, be informed that External data is a little bit different from external lists in the way it’s only copy of the external data but you can always refresh it manually.
I would also like to highlight another feature, you can now author SharePoint workflows that access external data through external lists or external data columns. Brilliant!