I’d personally go with WebHooks (or something similar). »Mitcho« already designed a plugin for that, called HookPress.
Webhooks are a simple paradigm for developing instant notifications and mashups based on simple HTTP requests. With HookPress you can set up webhooks so that a specified URL (a public service or something you set up) is requested when certain WordPress actions occur. Possible uses include generating push notifications or using non-PHP web technology to extend WordPress.