I'd like to have a nice large table for the various scenarios and browsers, but until then I'll just make my notes here.
AJAX Cross Site
GET allows only application/json, application/javascript, text/javascriptGET may have a body (thus emulating an AJAX post), except in IE
POST is forbidden
HEADERS forbid X-HTTP-Method-Override
CORS is supported by all modern browsers except Opera
HTML Cross Site
GET allows application/json, application/javascript, text/javascriptGET forbids text/plain
POST allows application/json, application/x-www-form-urlencoded, multipart/form-data
POST forbids none
Same-domain
Everything should be supportedHEADERS allow X-HTTP-Method-Override
http://annevankesteren.nl/2007/10/http-method-support