The new HTC Titan Windows Phone hit the AT&T online store yesterday and is expected to be in AT&T retail stores today. If you are wanting to check it out in person though, call ahead. All indications are that there will be a limited number of the phones available. This is mainly due to the low production of the devices but also AT&T is likely hedging their bets a little so they do not end up with mass inventory in their stores.
I would also recommend that you call on AT&T owned stores, not authorized resellers. The authorized stores likely will not have the phones at all unless that is your only option in your city.
To find the closest AT&T store to you, go to http://www.bing.com/local and search AT&T
If you want to order online, you can click the AT&T logo below.