Scott Hanselman has been having a conversation with Microsoft's Sean Lyndersay about the feed URI scheme (aka: feed:// protocol), specifically about why IE7 doesn't support it. Since I was among the RSS aggregator developers who promoted this scheme, I figure I should chime in here.
The feed URI scheme was invented to deal with the problem of mainstream browsers spewing XML when a user clicked a link to a feed. In theory, MIME types were supposed to help in situations like this, but in practice MIME types weren't a reliable solution. So, to get around this usability nightmare, many aggregator developers suggested using a feed:// pseudo-protocol for feed links, so that applications could register themselves as the default application for that protocol. This way, clicking a link to an RSS feed would subscribe to that feed in the user's aggregator.
Now, I'll be the first to admit that the feed URI scheme was never a perfect solution to feed subscription, especially since clicking a feed:// link would cause the browser to display a cryptic error message when no application was associated with feed://. But it did work well when an aggregator was installed, which IMO was an improvement upon spewing XML regardless of whether the user had installed an aggregator.
However, now that mainstream browsers do a better job with subscription, I'm less inclined to promote the use of the feed URI scheme. Firefox, Safari, Opera and (soon) Internet Explorer understand feed subscription, so clicking an RSS/XML button is a far more user-friendly experience than it used to be. Given this, I don't think it's a big deal that IE7 won't support the feed URI scheme. It would be nice if it did, but it's not a deal-breaker for me.
That said, I agree with Scott that it's odd that IE7 doesn't support feed:// given that Outlook 2007 does support it. And it's also odd that Sean's reasoning for not supporting it is because Microsoft believes aggregators should integrate with the Windows RSS Platform (in Sean's words, they want to "deprecate the notion of a single feed reader, and instead, promote the idea that feeds can and should be consumed from multiple different applications"). If that's the case, I have to wonder why Outlook 2007 doesn't integrate more closely with the RSS Platform. As far as I can tell, Outlook 2007 does support the common feed list, but it uses its own feed storage rather than that provided by the RSS platform. I'm sure they have sound technical and/or scheduling reasons behind this decision, but the fact that one of Microsoft's most widely-used applications doesn't fully integrate with the Windows RSS platform could make the platform less attractive to third-party developers.