I'm sure that the estimates are correct. In fact, they're probably on the low side of the possible range (anywhere from 48,000 to 55,000). The city limit signs themselves say something right above 48,000 - I'd drive out there now to see if I were at my father's, but I'm in San Antonio right now. Today is the third day of the semester, and I have the greatest professors.
Within another ten years I'm sure that the Austin area will have three more cities with 50,000 plus: Kyle, Leander, and one of either Taylor or Hutto (perhaps both). The town that will rapidly expand after that will most likely be Manor.
Also, Georgetown is actually smaller than Pflugerville (going off of each cities most recent estimates).
Georgetown is at 50,061.