We just published a patch release of the Workflow Power Pack for SharePoint. No new functionality, just a fix to deal with what we believe to be a bug in the way SharePoint 2010 deals with custom Workflow Activity Validators.
Due to the manner SharePoint 2010 deals with lookup fields in custom activity validators, any custom code that contains lookup values (e.g. [%Current Item:Transaction Date%]) will not be validated when a workflow is published. We believe this to be a bug in SharePoint 2010 as this works fine in SharePoint 2007.
If this is a big problem for you then consider passing in the lookup field as either parameter 1 or parameter 2 and refer to it using MyWorkflow.Parameter1. For details on how to use the MyWorkflow object in your own code, see the User Guide Series.
Code that does not contain lookup fields is validated properly. Regardless of the scenario, the code does execute correctly.
SharePoint 2007 environments are not affected by this bug.