If you're expecting the WF (Windows Workflow) passivation services to work out of the box, well, you're half right. 

From one of my workflows, I've been able to trap that an error was being thrown at some point during execution, but it wasn't clear what the actual error was or what was causing it.  I knew it had to do with passivation services since everything was just fine and dandy once I removed the passivation services from the equation.

From the runtime level, I received the following not-so-helpful error message:

System.InvalidOperationException: Workflow with id "[some-guid]" not found in state persistence store.

Just on a whim, I attached SQL Server Profiler to the database instance where the persistence store is attached to track the queries and lo-and-behold, the mystery culprit surfaced:

declare @p10 int
set @p10=0
declare @p11 uniqueidentifier
set @p11=NULL
exec InsertInstanceState @uidInstanceID='2686AB8F-35A4-49C3-AED8-9F239D00D3AC',@state=0x,@status=3,@unlocked=1,@blocked=0,@info=N'Type
''Zorch.Alta.EastCastle.Workflows.DataProviders.Implementation.MockStepProvider'' in Assembly ''Zorch.Alta.EastCastle.Workflows, Version=1.0.0.0,
Culture=neutral, PublicKeyToken=null'' is not marked as serializable. Type ''Zorch.Alta.EastCastle.Workflows.DataProviders.Implementation.MockStepProvider'' in
Assembly ''Zorch.Alta.EastCastle.Workflows, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'' is not marked as serializable.',@ownedUntil=''9999-12-31
23:59:59:997'',@ownerID=default,@nextTimer=''9999-12-31 23:59:59:997'',@result=@p10 output,@currentOwnerID=@p11 output
select @p10, @p11

Well, yeah, duh!  It seems so obvious after the fact.  So the moral of the story is to make sure that all of your classes are marked with serializable :P I'm just not sure why they decided to place the error message in the procedure call to insert the instance state.  Why not actually raise an error to the workflow runtime that a class was not serializable?