Ask HN: Cross-platform runtime for my project?

I'm building an app in which I need to implement a desktop server portion (not http). It also needs a pretty GUI, mostly for providing tray/dock notifications and viewing web-type content, so probably something with a lightly-wrapped webkit reading local files or talking to an embedded HTTP server. It's important that my source code for the server portion not be utterly trivial to extract. It needs to run at least on OS X and Windows. I use Linux day-to-day myself, so I'd like to not leave myself out, but it seems very likely that paying customers will be more concentrated on the other two OSes.

I know PHP and JavaScript deeply. I have experience with Ruby and Python. I have experience with Java but I don't especially like it nor think my clients will appreciate the heavy runtime.

I was thinking of writing the server in PHP and using phc to compile it. Any better ideas or alternatives? There's py2exe, and I believe it would be possible to bundle all the necessary runtime with precompiled python files in a mac .app, but I expect python bytecodes are not very difficult to decompile.

For the GUI, there's AIR (ick: proprietary), XULRunner, and Titanium (crashes when trying to do much on my system) in the JavaScript-y space. What else should I look at? Remember that I need high-quality HTML/CSS/JavaScript rendering embedded. (Sort of? In a pinch, I could do notifications, config, and a list view in native-ish chrome and call the default browser to actually view the content. In this case the browser chrome would be no benefit, though, so I'd rather have this all together.)