Hi Thomas, that's a really fantastic idea, thanks. If my airfare were covered it would be a no-brainer! I like the idea of doing a hacking day before/after as well.
In a previous post I mentioned several areas of OG I'd likely want to hack on as my project progresses, so any of those would make a good proposal, and of course, we don't have to stick to the script when we actually get together.
If you want you, you might want to submit a Hackathon proposal for YAPC::Europe: http://vienna.yapceurope.org/ye2007/cfh.html
We can provide sponsoring for travel and accommodation costs for up to 500 EUR.
The deadline to submit a proposal is Sunday, 13th May.
And if you want to tack on a day of hacking in Vienna before/after YAPC::Europe, I'm quite sure we can find a location with power plugs and WLAN.