There is one other step vitally important to make this 'synch' feature work. I concur that the CALLER ID has to be set to your Apple ID email address in your iPhone (and I don't think there's any other choice in the iPad for this) - but ALSO -- you must have a 'self' identity in your CONTACTS bearing the same APPLE ID email address in first position of all email addresses you have listed for yourself.
So make sure you have a CONTACT for 'yourself' in the iCloud synched to both iPhone and iPad running iOS5 and that the 'first' email (often marked as HOME) is your APPLE ID email address.
THEN - if you have the caller ID settings set correctly as described earlier in this thread - the conversations between all iOS5 people out there you're talking with in iMessage will appear on both iPhone and iPad running iOS5. To make it 'iron clad' - you need start a NEW conversation in the messaging window FROM YOUR IPAD first!. Conversations started (pre your making all these settings right between devices) already in your iPhone don't just magically transfer to the iPad. They 'should' - I agree - but they don't. Initiate a NEW conversation to a fellow iOS5 person FROM your IPAD first. THAT message SHOULD immediately appear in your iPhone and then you're good thereafter. At least that's how it's worked for me. It's pretty 'convoluted' and Apple should give VERY specific instructions for this but they don't.
Remember, too, that iMessage will not work when texting to non-Apple devices and people not converted to iOS5 yet. In that regard BlackBerry Message is WAAAY more advanced a texting technology. Still prefer my Apple any day. But iMessage won't really ever be fully practical until it can work to ANY phone or tablet device on any carrier.
Hope that helps.