The problem with the MtGox/Bitcoinmarket solutions is that they still require you to copy&paste an address if you want to pay directly from the bitcoin client running on your machine. My computer-phobic relatives just won't be able to do that (I can hear them saying "...BUT I DON'T HAVE A COPY BUTTON...").
But a bitcoin: URI has problems, too:
1. It is hard to implement-- every browser has a different way of registering protocol handlers.
2. If you don't have the protocol handler installed, clicking on the link doesn't give you any help on what to do to make it work. (try it here)
3. Looks like some software (like this forum) don't like bitcoin URIs: Donate to the Faucet (that URI is bitcoin:15VjRaDX9zpbA8LVnbrCAFzrVzN7ixHNsC, the forum software is truncating it and adding http://).
Maybe a bitcoin payment request MIME type would work better; you click on a link and the web server returns a little file of type "application/bitcoin-payment-request". Like this. Or the links on this page.
If you're running Firefox, you get this dialog box if it doesn't know about application/bitcoin-payment-requests:
On PCs, an app can add stuff to the registry to associate itself with a mime type (anybody know if browsers other than IE pay attention to those registry settings?).
Macs are similar (although I don't know the details; gotta set the mime type in the App's Info.plist, I believe...). Anybody know about Linux?
Teaching the Bitcoin application to read the bitcoin-payment-request file would be easy, and it shouldn't be terribly hard to get it to ask an already-running Bitcoin to popup a "Send Payment To..." dialog box with fields filled in (OR get it to start itself running and then popup the dialog box).
So: whaddya think?