Hmmmm. I had been complaining for months about my supposedly "50 Megabit" link from RCN Cable being more like 12. But I guess I should have actually complained to RCN instead of just to my friends: I called them tonight, and although I had to wait on hold for 20+ minutes, the guy reprovisioned my modem and walked me through rebooting it (slightly tricky since you have to remove the battery) and now I get a solid 50 Mbits.
And not just from the not-sure-I-trust-it speedtest.net: it can also deliver 3x 15 megabit UDP streams with near-zero packet loss using my VPN to work.
Wow. A little tech support goes a long way. Let's see if they can sustain it for a few days. :)