SMS can come in alongside phone calls. I’ve got used to this, I’ll sometimes call people while waiting for other people to turn up, I’ll hear a text message arrive, can read, and act accordingly.
iMessages are shiny. iMessages are data. This means they’ll only arrive if I’m on WiFi or on a 3G UTMS connection.
If I’m on a 2G GSM connection, or a CDMA network user in the States, and not connected to WiFi, I’ll not be receiving messages while I’m on the phone.
Yes iMessage says things are “delivered” but I don’t yet know what that means, and I’ve seen people complain on twitter that it’s not delivered to the handset. I guess people haven’t seemed that bothered with BBM, but that has stronger delivery feedback; But the transparency of SMS/iMessage is the problem, I’ve had people ask “HOW DO I SEND THEM?” because the only distinction is the blue/green colouring, in some ways it’s so transparent you might not understand.
I’m not sure this is a massive issue, but it’s got the potential to make meeting people comically awkward, given my phone still spends a bit of time on 2G networks.