To help, I've started a simple library that helps make our lives a little bit easier. It's nothing big yet, doesn't validate the data yet, and surely has lots of bugs. In other words: don't use it for production without understanding the potential glitches. If the data you are sending to the server is likely to have "<" or ">", be careful – the library will very likely barf. I'm sure there's things I haven't even started to consider.
Even so, it's a start, and it doesn't require any additional libraries. I can verify that it works from within PhoneGap (which was the main point), at least on WebKit browsers. Android 2.x may have a bug with the "content-length" header – in which case, just remove that line. I have no idea about WP7 or WP8. On iOS, it just works.
The gist is here, under an MIT license: https://gist.github.com/3786070
Feel free to fork, improve, use, etc. If you do find a bug or implement a fix, it'd be nice to share it back, but since it's under MIT, you're under no requirement to do so.