Since the rollout I've seen at least one or two messages regarding the altered responses we're now sending with the REST head of the service.  There are several reasons why we've changed the service to now not return the entity body back to you again.  These are as follows:

1. Version - Prior to this rollout the only mechanism that we had to communicate version information to you was via the entity body of the response (specifically the version element in the body).  However, now that we have true ETag support we now can simply return this value back to caller via the ETag header per the Http spec.
2. Cost - Although at this point we don't charge for the service we eventually will.  Included within this billing structure are charges for ingress & egress (bandwidth usage in essence).  Therefore, by not returning the body to the caller (which the caller originally provided us with anyway) this is a net cost savings for you the caller. 

Hope this helps to explain a bit more of the thinking behind the change.  As always we're open to feedback.

--Jeff--