This morning, I was seeing sporadic results (10-24Mbps) with the iPhone5 for the default Speedtest.net location. That might not seem bad, but this is FiOS service where I normally get 81-83Mbps on Speedtest.net with a wired network connection and 30-40Mbps via wireless on my iPhone4S. These results are with the Verizon-provided Actiontec MI-424WR (Rev.I) 2.4GHz 802.11n router.
This afternoon, I found that I could get more consistent results in the 25-30Mbps range with the iPhone 5 when using the NY Speedtest.net location rather than the default location.
This evening, I swapped out my Verizon-provided router with the widely recommended Asus RT-N66U wireless router. This router provides separate 2.4GHz and 5GHz wireless networks for different devices. Here's my iPhone5 Speedtest.net result to the NY Speedtest.net server using the new router.