The Short Version: Depending on the network environment, network authentication may block iMessage text messages. In addition to blocking messages to and from iPad and iPod touch devices, it will also result in blocking iPhone text messages. The simplest work-around is to disable iMessage in Settings -> Messages or just disable WiFi.
iMessage is a useful feature. It’s available in iPads, iPhones, and iPod touch devices running iOS5. Simply put, it provides the ability to send text messages over wifi and/or a cellular network without using SMS. It’s more useful for iPad and iPod touch owners with wifi-only devices. For example, someone using an iPod touch can send a text message to a person with an iPhone.
At first glance, it’s a great concept. It’s integrated into the default Message app. Unfortunately, some users may find it completely useless. Specifically, it’s a problem for individuals that connect to wifi networks that require authentication and also require continuous activity to remain authenticated.
I do. I’ll forgo the details of where or how but in order to access the wifi network from my iPhone I must login to the network using a Web browser on the device. I’m required to re-authenticate after a set period of inactivity, so I can’t just login once per day and be done with it.
This affects my ability to receive text messages via iMessage. That’s not to say I’ve never received a message without authenticating, but there were probably other reasons it seemed to work (perhaps my device had just switched over to the wifi network at the time or maybe I authenticated earlier and then forgot that I had done so).
This effectively blocks the usefulness of iMessage for some. To cause further problems, the iPhone seems to already know who does and does not have an iMessage compatible device. If I text another iPhone user it will default to sending the text message via iMessage.
In the Message app the iMessage texts are colored blue. Standard SMS texts are colored green.
To work around this problem I’ve switched the iMessage feature off (Settings -> Messages). This means that I can’t receive text messages from iPod touch or iPad (wifi-only) devices, which is unfortunate since my wife recently received a new iPod touch for Christmas. However, it seems to be a better compromise than disabling wifi.
Note that this isn’t a problem with network authentication methods. In fact, they’re working properly if they are blocking iMessage to unauthenticated users. I’m not sure Apple thought this through. Granted, most users may never deal with this problem on a regular basis, but Messages should at least be updated to automatically, and immediately, send a text via SMS from the iPhone if it can’t be sent via iMessage over wifi. We’ve come to expect text messaging to work almost instantly and somewhat reliably.
Updated 04/11/2012: Instead of disabling iMessage I’ve started turning off WiFi and use only the 3G connection. Obviously this is only a possible solution for iOS devices that have mobile data plans so it won’t work for an iPod touch.