I have always maintained an interest in workflow, and on a few occasions have been able to put it to use productively on some of my projects; from a good few years ago implementing WF as a web site process controller (see my post here on integrating WF with ASP.Net MVC), to a few months ago giving public presentation around the improved integration between WF and WCF in Net 3.5 (Send/Receive activities) at my current employer's regular community events.

Well, from reading the lead article in this month's MSDN magazine (and between-the-lines of some of the material released at this years PDC (start here for good overview)) it seems that when .Net 4.0 arrives WF may finally be reaching a level of maturity to justify a more mainstream role in the .Net world.

• There is a new WCF/WF integration model. Although only a year or so old, it seems the Send/Receive activities are to be little more than a stop-gap solution and will be replaced with new richer SendMessage/ReceiveMessage activities.
• The concept of WCF hosted workflows which was released in 3.5 is to be developed to a level it will now be possible to define both the workflow and the service and message contracts from within the workflow Xaml itself, reducing a good degree of redundant boilerplate code/configuration.
• The WF designer within Visual Studio is to be rewritten (hopefully withpout as many frustratingly erratic  bugs) to support this new functionality and to enhance the debugging experience of Xaml-only workflows.
• There is an update to the Server 2008 'Application Server' role (currently codenamed Dublin) in the pipeline which will add integrated management and tracking of workflow instances hosted within IIS.
• ...and probably most revolutionary of all - as the mechanism by which azure provides code execution in the cloud - hosted workflows may finally be the 'killer app' that forces otherwise ambivalent developers to take a fresh look at the technology as a means to solve their problems.

On the downside, though:

• The new flowchart workflow process model and various other activity execution model changes seem that they will require breaking backward compatability with custom activities written for .Net 3.5 and before. Although for those of you yet to make any significant investment in custom activities this may be hit you don't even notice.

Obviously much of this technology is at the announcement or early CTP stage, but I would seriously recommend reading the couple of articles in this month's MSDN magazine, downloading the PDC VS2010 CTP (including .Net 4.0), checking out registering for a trial account at the azure site and generally keeping your eye on WF over the next 12-18 months.