Buggy? it's in beta

That's probably why.
In terms of your messages 'not arriving' when you're not signed in, there is a reason for this; well several.
1) When an iPhone registers with the messages server, it first registers its number, which then the messages server associates with your email addresses as 'other means of contact'. if the primary means, i.e. your number, receives the message, as far as it's concerned, the message has been received.
2) When a non 3G iPad logs into the messages server, it then registers itself with your Apple ID. making the Apple ID the primary means of contact. Ditto for the rest in (1)
3) If both were offline and:
i) Person A were to message your mobile number
ii) Person B were to message your Apple ID
--Whichever device logs in first, because the messages server didn't register the message as 'received' it will broadcast BOTH to whichever logs in first (as the Mobile Number and AppleID are linked) ergo marking the message as delivered
4) If you have, 'Sync Documents' enabled on ALL of the devices, this will also sync the iMessage caches with one another. It won't remember if you've read it on one and not the other, it just remembers that 'its been read'
Anyway, hope this helps!
Cheers,
A