Your recurring payment suggestion is too simplistic.
It should be able to do trials and up to 3 steps of billing. (Most sites don't need more than 3, we could make it unlimited).
Example: 4 BTC for the first 7 days, then 100 BTC every 30 days after that until canceled.
There are lots of sites that really need to offer trials.
That sounds good... but even PayPal can't do that. Let's at least get something up and working now. I suppose additional parameters could be added as needed.
Like....
bitcoin:15kDhRAcpgsugmh6mQsTcCHdvbsuYncEEV?btc=4?=freq=daily?=endson=7?btc2=100?=freq2=monthly?=endson2=never?label=Arvixe?account=punl-9482054-3402?message="new trial subscription"?=processingdonation=0.02
Obviously, we'd have to all agree on the format.... because the apps would have to build in functionality to handle such complex recurring transactions. That's functionality that (obviously) does not exist in the clients today. So those extra fields would simply be ignored in older clients that can't handle recurring payments. Or clients that can only handle one level of recurring payment schedule. I've never seen any mainstream app that can handle more than one recurring payment schedule as a single entry. For example, PayPal can't do that. Online bank bill-pay services can't do that. They only do: PAYEE, AMOUNT, and FREQUENCY. That's it! Not even an end date.
So.... we have to start somewhere. Let's get the foundation built. We can worry about the flooring and drapery choices a bit later.
I love that satoshi mentioned the idea of making this URI into a QR Code way back in July or something... because I had the same idea. ALL the new phones apps scan a QR Code, that the QR Code displayed at a store's cash register could contain all the specific details about THAT transaction.... maybe even a unique Bitcoin address just for that transaction...? But for sure, an account number, invoice number, or some such number to automatically correlate your payment to your cash register sale.
Of course, online merchants' sites could automatically generate a new UNIQUE uri just for THAT SPECIFIC customer invoice also... all automated.