It is not possible to do an asynchronous
POST to a service on another domain, due to the (quite sensible) limitation of the same origin policy. JSON-P only works because you’re allowed to insert
<script> tags into the DOM, and they can point anywhere.
You can, of course, make a page on another domain the action of a regular form POST.
Edit: There are some interesting hacks out there if you’re willing to go to a lot of effort inserting hidden
<iframe>s and mucking about with their properties.