The most interesting part of the WWDC keynote address was not iCloud, even though that got all the ink. Most of the features included in that package are already being done by Dropbox or Amazon or even Apple itself (Mobile Me), so the fact that it’s getting dressed up in one spiffy new package doesn’t turn my head.

But I was very interested to hear that Twitter will be a system-level service in Apple’s mobile OS. Such a development should make creating a Twitter account dead-simple, and it hints at the ability for all apps — games, for instance — to take advantage of tweeting (not something I would necessarily use, but it will deepen attachment to the service for many). Both steps promise another boom in the number of Twitter users, taking the company that much closer to becoming  indispensable.

Why would Apple be interested in the arrangement? Dan Benjamin nailed it Wednesday on his podcast “The Talk Show”: The only weakness in Apple’s arsenal is social, while Twitter has succeeded only at social. Twitter, famously, has yet to monetize that growing audience (Promoted Trends, anyone?), while Apple, famously, is flush with cash.

Twitter says it has no interest in being acquired, but if that position should change, Apple now appears perfectly poised to execute the takeover. In fact, Benjamin and his co-host, John Gruber, suggested convincingly that Apple got a right of first refusal for any Twitter purchase in exchange for granting system-level integration. (Here’s the podcast; the relevant portion starts at 1:09:00 into the episode.)

All of this, combined with the TweetDeck deal, indicates that Twitter, at long last, has the security to forge its own long-term path. And that’s the real story out of WWDC.