Can I use iMessage without a SIM card??

You will need a SIM card and SMS service with a supported carrier to initially activate iMessage on an iPhone. iMessage activation on an iPhone requires an SMS message. Once activated it should continue to work with an iPhone with the SIM removed, although obviously not with your telephone number, but over wifi.

"iMessage" will work....but not "SMS" messaging.

iMessage will continue to show "Waiting for Activation...", but it will in fact, work and sync iMessages between all you devices.

BUT...

All you devices must be using your Apple ID in the 'You can be reached at' & 'Start new conversations from' area, to talk to each other (and sync all together). The only one that will still get any SMS messages, is the iPhone with an active SIM card. (The iPhone will activate the iMessage function due to the working SIM).

This does allow an old phone to be used as an iPod.