How likely is it that Siri comes to other iOS devices?

Apple (s aapl) is apparently trying out Siri on devices other than the iPhone 4S, according to reports circulating over the weekend. It comes as no surprise that Apple would want to see how Siri works on different devices, but how likely is it that those of us who aren’t Apple engineers will ever get the chance to use the personal assistant on anything older than an iPhone 4S?

No technical barriers

For late-model devices, there doesn’t appear to be any technical barrier really standing in the way of getting Siri working. The iPhone 4, apparently, can run it just fine with a few simple software modifications. And if Apple’s 2010 smartphone can handle it, it’s almost guaranteed that the iPad 2, and likely even the latest generation iPod touch can do the same. There’s no special hardware feature that the iPhone 4S has that distinguished it from those devices, so technical concerns aren’t preventing Siri from coming to more recent iOS devices.

Why Siri?

Why did Apple create Siri? That’s the question that may have the most to do with the possibility of seeing it on other platforms. There are a couple different motivators, and each lead to a different conclusion about its propagation to other devices:
1. Siri is a device-defining feature
The iPhone 4S has some changes under the hood that improve general performance, and a much better camera for mobile shutterbugs. But the feature that most distinguishes it from its predecessor is arguably Siri; or at least, Siri is the most marketable and obvious difference. Apple has proven it thinks this way with recent iPhone 4S ad campaigns, many of which focus entirely on Siri.
In that regard, Siri is a key selling feature for the 4S. If Siri is available on the iPhone 4, there’s less of an obvious reason for customers to upgrade to a more expensive iPhone 4S.
2. Siri is an iOS ecosystem perk
Apple has differentiated its iPhone line more than it usually does this time around, by keeping the 3GS for free alongside the iPhone 4 and iPhone 4S. So long as its profit margins are similar for each line, Apple may not care which it sells more of. In other words, if Siri on the iPhone 4 seems able to drive iPhone sales higher in general, Apple could be less concerned about keeping it as a device-specific feature. Still, the pressure from investors and public perception to make the latest device also the best-selling is considerable.
As an ecosystem advantage, Siri serves Apple best by making it to as many devices as possible. That way, it stands a much better chance of being yet another reason users are reluctant to switch to another platform. If this is really what’s behind Siri, Apple may choose to put it on as many currently sold devices as possible.

Siri is a beta product

Another thing to note is that Siri is listed by Apple as a beta product. The five-hour service outage last week brought that point home. While in beta, Apple is wise to keep the initial user pool relatively small by limiting it to just iPhone 4S users, compared to if it had launched on all current devices simultaneously. Once it gets the bugs ironed out, and brings its North Carolina data center up to speed, it may consider expanding the pool to other devices, without incurring too much risk of inconsistent, problematic service.

Likely outcome

I think it’s likely Apple will eventually bring Siri to other older devices, but maybe not all that are technically capable of running it. The iPhone 4, for instance, might be left out if Apple prioritizes sales of the 4S above older hardware, which I still think it’s apt to do despite its diversified smartphone product line. Putting Siri on other technically capable devices, however, like the iPad 2, serves the ultimate goal of making Siri a platform-defining feature, without immediately threatening the sales of more current hardware. Adding those devices will also let Apple scale the service at a moderate pace, whereas potentially adding the entire iPhone 4 user pool would represent a huge increase in demand on infrastructure resources.
What do you think? Will Apple freely spread around the Siri love, or keep the personal assistant’s magical pixie dust in reserve to boost the sales of future devices?