RE: hippich XML/JSON:
XML ain't gonna happen, and I think JSON is overkill for this. Using the same format as the bitcoin.conf file is the right answer, I think (easy to implement, human-readable, comments allowed in the file, easy to extend as necessary). That's what I mocked up at skypaint.com.
RE: nimnul http://localhost:.../
If you've never installed bitcoin, or bitcoin isn't running already, you get an unhelpful error message. One advantage of a "payment request file" is many users will understand that you need something called "Bitcoin" to open a "Bitcoin Payment Request" file.
RE: mizerydearia Linux support:
In my research I briefly an across a reference to a KDE startup file to associate mime types with applications, so I think it would work nicely cross-platform if you've already installed bitcoin. (UPDATE: cross-desktop spec here)
RE: nimnul clipboard monitoring:
Seems to me that's a feature for geeks, because only geeks would happen to know that if you highlight a bitcoin address when bitcoin was running you could automatically launch bitcoin. And there's no way that would be turned on by default. And it also seems to me clipboard monitoring is perfect for a standalone, general-purpose utility ("if selection matches any of THESE regepxs, launch THAT applications with arguments "beginpayment %{address} %{amount}" ... ).
RE: mizerydearia portable devices:
I've never programmed for Android or iPhone; anybody know how they handle non-standard MIME types? Can apps register themselves as content handlers?
Thinking about actually implementing this, the hard part is getting a running (or just launched) Bitcoin's attention in a cross-platform way. Using the existing command-line control (which uses JSON-RPC under the covers) would be the most straightforward, but that requires that you set a rpcuser/rpcpassword in the bitcoin.conf file AND run bitcoind or bitcoin -server. Yuck.
I suppose, as a special case, payment request JSON-RPC calls could be allowed with no rpcpassword, and -server could be made the default. Makes me a little nervous, though.
Running bitcoin /path/to/browser/cache/with/payment_request_file (which is what the web browsers will do) should:
IF bitcoin isn't running: launch it, and wait until it is ready to receive payment request messages
Send running bitcoin (...somehow...) "Hey, we got a payment request here for you to deal with" messgae.