iPhone can send iMessages without a SIM card? I gave my old iPhone 3GS (iOS 5.0) to a friend, and after removing the SIM card, we noticed a peculiar behavior. My friend is able to send texts via iMessage from the old phone number. Likewise, I can receive texts from this number.

Has anyone else noticed this? I am assuming the iMessage servers register phone numbers for compatible devices. But what happens if someone else claims this number?
iMessage can also register email address (like your Apple ID), that's how it works on iPad and iPod Touch.

So it's completely normal to be able to send iMessages with an SIM-free iPhone.

As long as you are connected to a WiFi network.