According to Reuters, T-Mobile USA is having the best luck with number portability thus far. In the last quarter, the company added 1M subscribers--more than Cingular Wireless and AT&T Wireless combined. This brings the total to over 13M. T-Mobile UK posted similar numbers, but didn't have to contend with the new pressure of LNP.
Verizon Wireless is expected to announce strong numbers as well.