If voice-activated user interfaces like Siri in the new iPhone 4S really take off, third party application developers are going to want in on the action. As John Gruber points out in his iPhone 4S review, that poses an interesting set of problems:

People are going to start clamoring for third-party Siri integration as soon as they see Siri in action. But I’m not sure what form that integration could take. Best I can think is that apps could hook up to (as yet utterly hypothetical) Siri APIs much in the same way that Mac apps can supply system-wide Services menu items. But how would they keep from stomping on one another? If Siri supported third-party apps and you said, “Schedule lunch tomorrow at noon,” what would Siri do if you have multiple Siri-enabled calendar apps installed? This is similar to the dilemma Mac OS X faces when you open a document with a file extension that multiple installed apps register support for.

And here’s a specific example of what he’s talking about, that involves only the built-in applications:

Here’s an example. Wolfram Alpha has terrific stock-price information and comparison features. I link to them frequently for stock info from Daring Fireball. So I tried asking Siri, “What was Apple’s stock price 10 years ago?” But once Siri groks that you’re asking about a stock price, it queries the built-in Stocks app for data, and the Stocks app doesn’t have historical data that goes back that far. “What did Apple’s stock price close at today?” works, but asking for historical data does not. But Wolfram Alpha has that data.

Working around those sorts of problems is difficult with regular touch or point and click interfaces — it’s easy to wind up in Preferences Hell. Dealing with them at the voice level is going to be even more complex.