Unfortunately, you can't control the on/off of the PS3 with just the USB-IR dongle. The dongle requires the power from the USB in order to receive IR commands. There is, however, a recent solution by Schmartz that provides on/off control by attaching a sticker to the on/off touch-sensitive button; however, the cost is $50. My understanding is that the Schmartz device uses a different code-set than the Nyko device.
Since I am usually inserting a Blu-Ray disk anyway when I'm starting the PS3, the lack of "on" control by remote hasn't bothered me. I just wish that Sony would add an option for the PS3 to automatically power off after a period of non-use. I tend to forget to turn the PS3 off.