FurryNutz -
Slow speeds led me to check traffic stats, and I saw that there was a high amount of traffic. I then looked at the list of client devices, and saw that there was a wireless client I didn't recognize. That's how I figured I got hacked.
What devices do I have using WPA? None. Which is why I'd prefer to disable it. Am I sure that the hacker was using WPA? No, because I couldn't see anywhere on the router where I could tell which encryption scheme a user was using.
While I know, as GreenBay42 pointed out, that WPA2 has security flaws as well, I'd still rather not make it trivially easy for someone to steal my Wi-Fi, which is the case with WPA enabled.