you can't have imessage functionality on iphone unless you upgrade to ios 5 or above. But, keep in mind that once you upgrade to ios 5 then ur baseband/mordern firmware will upgrade to 4.11.08, which will make your iphone unlockable.

you should be able to send imessage without sim card. Imessage is available on ipod touch too. I believe you have to activate phone with sim card and then remove that once activated.