iMessage message routing I had a question on how iMessage message routing works.

Assume this setup:

Thing1: 
iPhone 555-555-5555
Apple ID: thing1@me.com

Thing2:
iPhone: 999-999-9999
Apple ID: thing2@me.com

If Thing1 sends a new iMessage and selects Thing2's Cell as the destination, the iMessage appears only on Thing2's iPhone.

If Thing1 sends a new iMessage and selects Thing2's email address as the destination, then the iMessage appears on Thing2's iPad and iPhone (and at this point, there's two separate conversations from Thing1 on Thing2's iDevices).

I had assumed that since the Apple iMessage servers know that Thing2 can receive iMessages at both their iPhone and iPad (via their Apple ID) that it wouldn't matter if the iMessage was addressed to either cell or email (and that iMessage would route it to all associated devices).

Is that not correct?
You are assuming Thing2's iPad and iPhone are both acting as 'Thing2'. That is only partially true. Assuming the iPad is WiFi capable only, it can only receive messages on its e-mail addresses (the address of the iTunes account or other e-mail addresses). The iPhone, however, can receive messages at its phone number or linked e-mail addresses.

Now when an iMessage is sent to a phone number, the message will only appear on the iPhone as the iPone is only capable of receiving those iMessages. The phone number is linked to the iPhone, not to the iTunes account.