Erik Meijer goes into more detail in an OOPSLA paper called "Democratizing The Cloud" (pdf):
We pick the .NET Common Language Runtime (CLR) as our universal computation model. We prefer, of course, to use the already available CLR implementation on each respective tier: SQLCLR on the data-tier; regular CLR on the middle-tier; and Silverlight for Web-clients, or the regular CLR for desktop clients.
To split the program between client and server tiers, you flip a switch and add the "[RunAtOrigin()]" annotation in the source code to indicate which classes that you want to run on the server. Then just rebuild and poof, all the RPC/Web service stuff is taken care of for you. This is one area where Volta differentiates itself from GWT. In GWT, the client/server split is explicitly written into the code by the programmer. Another difference is that remote calls in Volta are synchronous by default but GWT's remote calls are asynchronous by default. Asynchronous calls require a little extra code to implement but result in a better experience for the user.
By setting another option you can have your Volta application cut trace records for later analysis in the WCF Service Trace Viewer Tool. This tool gives you an easy way to view, group, and filter traces so you can diagnose and repair issues in a multi-tier environment.
Microsoft labels Volta as "experimental" at this point so there are a few rough edges. For example, every class used in your application will be a separate download from the server. This means currently you can expect Volta apps to be much slower than GWT apps, at least the first time you run them. When I tried the online WordWorm sample, Firefox 2 put up a dialog saying the script was unresponsive (see above). On Internet Explorer it worked but took over a minute to start up. According to Microsoft they plan to make this smarter in future versions.
Analysis If Volta had been released two years ago it would have been revolutionary. At this point, though, Microsoft is playing catch-up with Google and Adobe. Volta also sends a confusing message to .NET developers targeting the browser. Silverlight 1.1 is supposed to include a full .NET environment inside the browser for multiple platforms. Now with Volta you can get "the illusion of" the same thing without a plug-in. So why do you need Silverlight?
Another concern developers will have is Microsoft's commitment to browsers other than Internet Explorer. If, for example, a new browser or operating system came out that broke GWT, then the source code is available so you could fix it yourself if Google's GWT team wasn't fast enough. That's not an option with Volta.
The Volta technology preview is available for download now at Microsoft Live Labs. The current version supports Internet Explorer and Firefox only.