After I heard it the first time I wasn't sure if I heard it correctly or not, but after a quick Google search I found the commercialy green blog dissecting and analyzing the commercial.
Commercially green was not able to transcribe the supposedly Czech line, so here it is:
Jak sa máš, brácho?
Which in casually spoken Slovak means:
How are you, brother?
The same thing in Czech would be:
Jak se máš, brácho?
It might be a small difference for Americans but I caught the difference right away.
Maybe if the director of the commercial read the book, The Foreigner's Guide to Living In Slovakia, he'd know that there is a difference between the Czech and Slovak languages.
The only explanation that AT&T can get away with is that there are a lot of Slovaks studying at Prague's universities, so it's possible that the "brother" has Slovak room-mates from which he picked up some Slovak. :)