I too have been with AT&T since the Cingular merger and am also not in the HIBP db. I use a custom email address related to Cingular, which I otherwise would have forgotten existed.
I've been with AT&T forever, I've used a custom email address for AT&T since forever, and my email address is also not in the dump (I have a Premier account.)
Same here. My 2004-era Cingular-specific email isn't in HiBP.