Later this summer, Signal for iPhone will be expanded to support text communication compatible with TextSecure for Android.

You'll have to wait to wait a little. I'd bet it will be worth.

Can a Stingray intercept iMessages?

I'm not sure, I doubt it, but that doesn't mean there aren't other ways to intercept them.

iOS still sends normal text messages to any device that isn't iOS. I use an Android device for the most part and almost everyone I know uses iOS, which means that even if I'm using TextSecure the message will be delivered as a normal text message, which certainly can be intercepted and stored by Stingray and similar devices.

No, the messages are encrypted device-to-device with Apple providing a key directory service. Apple must be complicit to read messages without physical or backdoor access to the device.

http://blog.quarkslab.com/imessage-privacy.html

This has nothing to do with the Stingray devices though.

Apple can certainly decrypt any iMessage, as explained very thoroughly in the article you linked to.

If Apple can decrypt them, then law enforcement can decrypt them, so they don't really even need the Stingray device.

Nobody knows whether or not the NSA stores all iMessages. They certainly can, and from everything we've seen if they can, they do, so if you are concerned about privacy you have to assume that they do. The thing about the NSA databases these days is that they allow multiple governments and other government agencies to access that data.

Imagine how many peoples lives would be "ruined" if someone took that data and published it on the internet, "here is every iMessage for the past year".

IMessages are supposed to be encrypted, so no.

It uses only data