<p>I'm looking for tools that implement the distributed agent/service model ... I guess we could also call this grid or cloud computing, but I'm not sure the term is exactly analagous.  A distributed agent model would be one where developers build agents that are dispatched to a framework and the framework executes the agent somewhere in the distributed environment.</p>

<p>Specifically, I'm looking for a framework that support dynamic distribution across the grid/cloud, but replaces the transient agent with a more persistent service.  The goal here is to minimize the amount of time and effort it requires to deploy a service into the grid/cloud container.</p>

<p>FWIW, I think Digipede gets pretty close to this, but I'm interested in what else is out there.  I'm under the impression that assembly distribution would have to be considered in order for the solution to be viable.</p>

There's a project at Microsoft called Dryad. Here's a good podcast with more info:
http://www.dotnetrocks.com/default.aspx?showNum=378
It uses LINQ and is supposed to be easy to use. Microsoft has it doing some production stuff for Microsoft's AdCenter.
Here's the research page.
http://research.microsoft.com/research/sv/Dryad/
Dryad is quite expressive. It completely subsumes other computation frameworks, such as Google's map-reduce, or the relational algebra. Moreover, Dryad handles job creation and management, resource management, job monitoring and visualization, fault tolerance, re-execution, scheduling, and accounting.
P.S. I don't see a download, but if you contact them and you are serious you could probably get it.
Update: it's now available for download here:
http://research.microsoft.com/en-us/downloads/03960cab-bb92-4c5c-be23-ce51aee0792c/default.aspx