Yes, but while on a call on a CDMA iPhone, each iMessage is sent with a 5 minute delay. I've tested this exhaustively. If after 5 minutes, the message does not send, the message is automatically converted to SMS and then you will receive it while on the call.
This does however make it impossible to carry on a text conversation with someone while on the phone (i.e. while on a conference call and not actively engaging in conversation) since each message tries to send as an iMessage before reverting to SMS after 5 minutes. Therefore, I've disabled iMessage on my CDMA Verizon iPhone so that I get my messages as texts without any delay. Personal choice.
This isn't true. Just tested this with my buddy on his 4S on Verizon.
It's true you get a 5 minute delay......on the first initial message sent.
1) Verizon user talking on the iPhone, WiFi OFF.
2) Me on my iPhone sends a text message to him. It defaults to iMessage because we are both on iOS5.
3) The message gets sent, but I don't get a delivered notice. The message just sits there in a blue bubble.
4) Verizon user doesn't get the message.
5) 5 minutes pass, I'm assuming Apple's servers sent a signal back to my phone, to resend the message as regular SMS. My sent bubble for that message turns to green and under it lists: Sent as Text Message
6) Verizon user gets it immediately and responds back.
7) I get that text on my phone and respond back to him.
8) Verizon user gets it immediately.
9) We text back and forth in real time, however now using SMS.
Basically, once the text was resent as SMS, and they responded back.....our conversation switched to SMS messaging. The Messaging app kept us on SMS, it didn't try to switch back to iMessage.
Once I had him turn his WiFi back on.....the next message sent automatically switched back to iMessage.
So the biggest downside for Verizon users is the initial 5 minute delay.
Here's hoping Apple reduces that down to 2 minutes or so.
-Kevin