Developers expect that the user agent will show the right image as the browser changes dimensions (e.g., it is rotated). However, the currently srcset spec says that user agents are /not required/ to do anything in response to changes to the browsing environment.
We need to get clarification from WHATWG/Apple on why their spec is not fully "responsive" (i.e., why it does not say "User agents MUST run this algorithm", but instead says "User agents are not required to ever run this algorithm").
Filed request for clarification from HTMLWG.
From my perspective of view it does not make sense to change an image that is already loaded unless it features an other crop. So it has to be specified by the creator that this happens.
Otherwise it is unlikely that you resize your browser on a mobile phone or tablet but for rotating. So it could also be specified to change on resize. Not an easy decision regarding the data bandwidth.
@anselmh, there is not much we can do about bandwidth.
The cases we need to pursue are any/all related to media queries.
BTW, there is a good article that discusses image resolution, compression, and pixel densities on HTML5 rocks with plenty of examples.
Ted filed a bug for us bugzilla. If you are interested, please add yourself to the CC there.