I've been playing with the REST protocol in SP2010 to bring in data from external lists (external to the site collection) and using it to display the result in the data view web part (DVWP).
Creating the data source went smoothly but when I tried to use it in the DVWP it threw the following unhelpful error.
"The server returned a non-specific error when frying to get data from the data source. Check the format and content of your query and try again. If the problem persists, contact the server administrator."
I checked the service in the browser and it was working fine - so a quick investigation identified authentication as the problem. I went back to the data source and forced the service to use my domain account instead of the default windows authentication provider.
As soon as the authentication was forced and I refreshed my data connection - the DVWP pulled everything through perfectly.
This issue seems to be with SP Designer not recognising your domain account when using windows authentication. This work-around will do the job - although it must be pointed out this method sends your credentials unencrypted in the call. Not the type of thing you would put into a production environment but will at the very least get you up and running with REST services inside SharePoint 2010.
More to come on this topic soon, specifically around potential uses of the service and a detailed walk through (time permitting as always!)