"Free Verizon Message: Your phone will soon be upgraded to Android 4.0. At that time we will remove your Verizon Wireless Mobile IM app because it is not supported on Android 4.0. Please download a new instant messaging app to use IM on your phone. Thank you for using Mobile IM!"
They didn't indicate a substitute (possibly not wanting to risk a lawsuit), but there are plenty of free instant messaging clients in Google Play. They include such free apps as Trillian (4.3 stars), eBuddy Messenger (4.3 stars), and imo Instant Messenger (4.4 stars).
Frankly, we never used the Verizon Wireless Mobile IM app, anyway, preferring Trillian, so it's not really a big deal to us. What is a bigger deal to us is getting Ice Cream Sandwich at all.
Apple had it right when they hammered Google over the small number of devices that have been upgraded to its newest Android version. On the other hand, considering the huge number of Android handsets vs. the small number of iOS handsets, it's like comparing Apples to oranges (pun intended).
They also didn't indicate when, but it could be any time, now. We'd expect that customers won't see the update over the weekend - less TS personnel around - but we could be wrong.