Sometimes, if you know where to look, you may be able to catch a hint at what Apple (Nasdaq: AAPL) has up its iSleeve next.

Some keen observers have noticed a couple of job postings in Cupertino for iOS software engineers destined to join its team to develop Siri, Apple's virtual assistant that stands front and center in its iPhone 4S sales pitch. Some of the wording in the postings suggests the possibility that Apple will open up Siri's application programming interface, or API, to third parties.

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One of the job postings for a senior position states, "You will have several clients of your code, so the ability to formulate and support a clear API is needed." The other posting says that "strong API design is needed to keep communications ideal." This is far short of definitive proof, since "several clients" could refer to internal clients or some of the external data sources that Siri taps into, such as Wolfram Alpha and Yelp.

Keep in mind that the Siri we all know and love today is currently still in beta, so the finalized version could differ in many ways. Apple needed to get Siri out the door to upstage Google (Nasdaq: GOOG) Android's upcoming version 4.0 Ice Cream Sandwich. Even after Google and Microsoft (Nasdaq: MSFT) downplayed the threat of Siri, Big G has highlighted the "powerful voice input engine" in Android 4.0 while Mr. Softy is introducing voice-controlled Kinect.

Opening Siri up to third-party app developers would tap into its revolutionary potential. Imagine asking Siri to "find me something funny to watch on Netflix" or "search for a Mother's Day gift on Amazon" through their respective apps. Ambitious hackers have already cleverly commandeered Siri into controlling TVs -- checking what's playing on different channels and channel surfing on your behalf -- and turning cars on and off.

While most developers have little reason to go through the trouble of setting up a proxy server that pretends to be Apple's servers to implement these functionalities, officially supporting and opening Siri's API up to third-party developers has limitless possibilities.

The App Store has already made the iPhone into a digital Swiss army knife; combining that with Siri's capabilities -- currently powered by Nuance's (Nasdaq: NUAN) voice recognition engine -- would truly be a force to be reckoned with.

Add Apple to your watchlist to see if it opens Siri up to developers. Get access to this free report on one stock set to cash in on the mobile revolution.