I've seen this problem on all distributions I've used. Most recently, I installed a 2012-09-18-wheezy-raspbian image, and the problem seemed a bit more pronounced than it had ever been. I almost always use ssh, so it's been ignored up till now.
I updated (apt-get dist-upgrade) to get the latest packages and used rpi-update to get the latest firmware and kernel, but the problem was still there. I searched the wiki, which suggested this is caused by power-supply issues.
So, I connected a 2200uF capacitor across 5V and GND, and the problem is solved. Looks like I've had a borderline power issue from day one. It was possibly more pronounced because I started overclocking, or perhaps the configuration of the GPU side has changed the power draw slightly.
I spliced the power-cable and soldered in the capacitor. It may also have worked had I just moved the keyboard closer to the USB dongle.