I've run into a few problems with some of the web apps I've been building lately as I'm sure others have. The explosion of the cloud is only going to lead more and more people down the exact same path. Let me just briefly explain the architecture which will help you understand why it's a problem.
Ideally what I want to be able to do is to push all this static stuff somewhere fast and as close to the user as possible. CloudFront is an obvious choice, but then I get hit by the same origin policy. To work around the issue and because I want to publish the API for developers to use I've implemented JSONP callbacks.
Though I did come across the W3C access control work and noted that we may be close to a solution, at least for the newest browsers. That brought up a whole new question that's been plaguing web designers and developers for years - how old does a browser need to be before you take it out back and shoot it?
Look through browser market share and my tiny sample set I'm also seeing that browsers are more or less up to date. Which is great news. There are a few IE6 hold outs showing up my visitor stats but for the most part everyone appears to be more or less up to date. Has software update actually solved the issue, probably not by I for one, welcome any progress.
So that brings us to the real question, with the vast majority of the browsers out there running at or beyond the most current release do we need to care about those that don't? Do we even want to deal with the train wreck that is IE6 compatibility anymore?