Did you ever find an answer for this? If so I would love to hear it, because I don't think it exists and I want it desperately.
Here is what I have encountered while searching for the same thing:
1) Adobe Air: Supports Windows and Mac. Dropped Linux support after version 2.?. Supposedly, if you avoid 3.0 features you can still use the previously distributed LInux version.
2) AppJS.org: NodeJS + Chromium. Supports Windows, Linux, and Mac (32 bit only). The lack of 64 bit support kills it for me. They say they are working on it, but I fear that they are just waiting around for Google to add 64 bit Mac support to Chromium. No idea when that will happen.
3) Titanium Desktop: Abandoned by Appcelerator to the desert of open source. Hardly usable right now and no guarantees about future.
3) XULRunner: Sounds promising, but I encountered a build bug on Mac that stopped me cold. I'm going to go try the latest release right now.
4) Python + Tkinter: Although this is a python solution and not HTML5, this is what I am using right now. Combined with Py2App and Py2Exe, it seems like it might work.
Overall, I am extremely frustrated at the state of things right now.