Well...I wasn't going to post this...I'll probably regret it, but what the heck. Filed a NASA ASRS report already. Background: I frequently make flights to Eloy, Gila Bend, and Coolidge after work in order to build hours. These are routes I am very familiar with and I do the VFR Bravo transition several times a week. I am genuinely comfortable operating the in the Class B around here. Departing KDVT I contacted KPHX Approach and asked for the VFR transition to the south. I normally specify my destination (Eloy, Gila Bend, Coolidge) as KPHX will typically give me Flight Following automatically (or sometimes they will ask if I want Flight Following as well) in addition to the VFR transition. This time I did not specify my destination - just that I wanted to do the transition to the south. I was given a squawk code, cleared through the Bravo, and assigned the East transition at 4500 feet. Just after passing Phoenix SkyHarbor and completing the Bravo Transition (just past the SFC - 9000 foot Bravo shelf) the controller stated something like, "Cherokee XXXXX, leaving Phoenix Class Bravo airspace, squawk VFR, frequency change approved." I acknowledged, "squawk VFR, frequency change approved, XXXXX". Shortly after, I began a climb to my cruise altitude of 5500 feet, not paying attention to the fact that the Bravo shelf extended down to 5000 feet until further out (about 10NM south of PXR VORTAC it starts at 6000 feet). This whole situation was sort of ironic considering I am normally very cognizant of the Bravo airspace and make sure to stay well clear of it unless I am doing the Bravo transition or have a Bravo clearance. At some point in the 5000-9000 foot Bravo shelf, the controller asked if I was still on frequency. I affirmed that I was still monitoring. At that point he let me know that I was in Class Bravo airspace (at 5500 feet) and needed to descend immediately. It caught me off guard how "nonchalant" he was about it. He was actually very friendly and professional about it. He then told me I could contact KPHX Approach on 123.70 (which I am very familiar with) if I wanted to pick up Flight Following. So obviously I booked it down to 4500. Took the wind out of my sails the rest of my flight, but oh well. It was really an honest mistake. Don't really have any excuses...wasn't distracted, etc. And that was that. So when I got home I filed the NASA ASRS report.