Will we mark Friday, April 9, 2010, as the day Twitter’s downfall began? That’s the day Twitter bought the iPhone app Tweetie with the intention of rebranding it as Twitter for iPhone. What’s the big deal, right? So what if they buy an app to avoid reinventing the wheel – it’s smart to take advantage of someone else’s work and carry on the legacy themselves. Heck, Microsoft and Google have been doing this for years.

But it’s not that easy for Twitter.

It’s not easy because there are thousands of developers that have been given access to Twitter and been told they can build anything they want with Twitter’s data as long as they obey some simple rules. And they have developed in droves. There are thousands of Web sites, desktop programs and smartphone apps because Twitter opened the world up to eager coders.

Then Twitter buys the Tweetie iPhone app and says, “Keep doing what you’re doing, but you’re competing with us now on the iPhone. Oh, by the way, we’re not charging for this app anymore. It’s now free.” Great for consumers, bad for developers.

Why shed a tear for developers? They’re the ones who’ve extended Twitter beyond its own borders and have arguably been a cornerstone of Twitter’s success. Twitter’s move is being felt in coding circles as biting the hand that feeds it.

Where do we go from here? We’ll certainly see slower development of Twitter iPhone and Android apps. Not so much on Web sites since they fill many holes in Twitter’s own service. But in the back rooms and garages where new Twitter apps are hatched, there will be discussions about how to compete with the mothership if Twitter decides to fill its own holes.

This may have implications for you and me as Twitter users. Will Twitter use slow as the tool development slows? Third-party tools have made Twitter both easier and more featured. Fewer tools could slow the use and growth of Twitter as we move into the future.

What are your thoughts? Do you think this is a strategic error? Or do you think Twitter is stronger for controlling another app critical to its ongoing success? The story is still being written, so add your chapter in the comments.