One of the more obscure new Twitter provisions are the requirements that the "@username must always be displayed with the '@' symbol" and that they must "link @usernames to the appropriate Twitter profile." This has mostly been taken to mean that awesome tweet-aggregating sites like Favstar.fm can't link internally anymore, and have to link out to Twitter whenever they use an @username. (Update 6:30PM EST: Twitter clarifies — Favstar (and others) do not have to link out to the user's Twitter profile directly, but it can link to "internal profile for the user within [its] app. That profile must have a link back to the user’s profile page on Twitter.com," per the footnotes here.") But read more closely, the intention of this guideline becoming a requirement is for Twitter to take more complete ownership of the @username convention in its entirety — to make it abundantly clear that it's a Twitter thing, and to make a person's @username totally canonical as a form of identity. In other words, Twitter wants @mattbuchanan to not just link to Twitter, but to mean me on Twitter, even if it's interpolated by another app or service. The problem, in part, is that if my friends or I have different @usernames on Instagram — particularly if we push those Instagram photos to Twitter — it breaks that convention, its meaning, and dilutes "the Twitter experience." In other words, don't be shocked when this tiny provision has far-ranging consequences for other apps and services, though it's not clear exactly how Twitter's going to go about sorting it out.

The thing about @usernames isn't entirely obvious, and it's only one of a number of ambiguities in the new rules. But that's fairly intentional. Twitter's not going to kill any apps or services that it actually likes, doesn't see as much of a threat or that it thinks adds value to the Twitter ecosystem, even if it looks like it's breaking the rules. So Prismatic and Rebelmouse may escape scrutiny, if only for a time, while Flipboard and Tumblr should get sniped relatively quickly. And in general, if Twitter product team director Michael Sippey named an app or service in this recent blog post, it's safe for now, though it may need to tweak things — Favstar.fm and Klout, for instance, will probably have to change the way they display tweets and links because Twitter wants to have complete control over how every piece of Twitter content is shown, all the way through. (You know, like a media company.)

Still, the consequences of Twitter slowly illuminating what its rules really mean sucks for a lot of developers. It's easy enough to say that the One Rule to Rule Them All is, "Don't compete with Twitter." Which is fine and dandy, because it's all for the greater good of Twitter or whatever. The problem is that it's real easy to be a valuable, contributing member of the ecosystem today and then tomorrow find out that you're now competition waiting to be crushed. Just ask the Twitterific guys.