UPDATE 2: It seems we were on the right track with this. Google employee Dori Storbeck has confirmed SMS integration is coming soon, as well as outbound calls. She revealed the nuggets when responding to questions from users on Google+ (spotted by The Verge) but didn’t go into specific details so it is not yet clear which platforms will get the features:

…yes, SMS integration is coming soon – it is one of our most requested features!

Future versions of Hangouts will support outgoing calls along with other Google Voice features.

(Original post continues below)

Support for SMS was one of the notable omissions from Google Hangouts, the cross-platform messaging service that launched at the Google I/O event yesterday, but it appears the absence may only temporary according to details within the Google Play store.

Included within the list of permissions for Hangouts for Android are requests to read and send text messages (as Google+ user Kenny Strawn noticed). That Google is already asking for permission to do these things is an indication that support may be coming, since the app doesn’t appear to use a device’s text messages at this point.

Here are the key permissions listed under ‘YOUR MESSAGES’:

READ YOUR TEXT MESSAGES (SMS OR MMS)
Allows the app to read SMS messages stored on your device or SIM card. This allows the app to read all SMS messages, regardless of content or confidentiality.

SEND SMS MESSAGES
Allows the app to send SMS messages. This may result in unexpected charges. Malicious apps may cost you money by sending messages without your confirmation.

This is an important point as it is unclear as to whether Google has decided against supporting SMS, it is unable to do so or the feature is still being developed.

Support for SMS would put Hangouts on a par with Apple’s iMessage service. iMessage allows users to send free messages to other iMessage users but switches to SMS when the recipient is not an iMessage user (or is offline).

Though iMessage is not cross-platform like Hangouts — which is available for iOS, Android and on the Web via the Chrome browser — the SMS-in and -out features allow users to stay connected with anyone on any phone, operating platform and location.

While it isn’t clear how Hangouts could support SMS, or whether it might be limited to mobile only, the ability to contact non-users from inside the app would be an important one that could be key to making the service usable on a regular basis.

There an alternative suggestion that the service uses SMS for other reasons. The app does use SMS to verify a user’s phone number, but that doesn’t appear to require these permissions since users manually enter a PIN code that is sent to their device via SMS. It could, of course, also be for other verification methods.

Google decline to provide comment on its plans for SMS in Hangouts, or what the permissions specifically cover.

We’ll leave you to consider the significance of that, but the fact that the company hasn’t dismissed this out of hand could be taken as a sign that the permissions may cover future developments.

Update: Google is actually already using SMS in Hangouts in emerging markets, as TechCrunch explains:

Google tells us that in some countries, feature phone users, but not smartphone users, can participate in Hangouts via SMS. This should help it reach more people in the developing world, a core area for growth of messaging apps.

That could explain why it needs to read (and receive SMS messages) on smartphones — when feature phone users connect — but it doesn’t explain why the Android app would send outbound text messages.

Hap tip @ow