Answer: To get my Roku HD 2000C player to communicate wirelessly with my Actiontec GT724-WG router I had to restrict the router to using WPA protocol (not WPA2).
Discussion: I've had a Roku HD player for several years and I recently purchased a second one. The new device would not connect to my wireless network. I called Roku technical support and we worked through multiple scenarios. We discovered that it worked perfectly wired and that it worked wirelessly if I turned off encryption on my wireless network. Changing the radio channel had no effect. Changing the WPA password had no effect. After about 1 1/2 hours of trying, support suggested I give it a couple of days and see if it would start working. And it did. For a week. During this week everything was fine, then it would not connect. I called technical support back and went through many of the same tests again. But no joy - it would not connect. At this point we thought that there was something wrong with my Roku device so support gave me an RMA number and I shipped it back for replacement. In a week I got a new Roku player - and it would not connect wirelessly. Talking with my brother-in-law gave me the idea that maybe my router's firmware was out of date. I checked the Actiontec site but I had the latest version. While poking around in my router settings I noticed that it was set to WPA/WPA2. Maybe WPA2 was the problem?? I set the router to use only WPA and voila, problem solved.
Note: During this whole time my original Roku player worked flawlessly. I'm guessing that it's old enough that it doesn't try to use WPA2 but I don't know how to verify this.