Something to watch out, well for me since @o2 changed their policy on sending MMS is that an iMessage can go as a SMS/MMS message out of your allowance if it is unavailable. This only applies to the iPhone (actually I don't know about iPad with 3G) but you can turn this feature on or off in within Settings > Messages > Send As SMS (see image below).
Apple support doc: http://support.apple.com/kb/HT3529