Just thought I would share what worked for me, since I had the same problem.
It turned out that to get this working, I had to use a specific USB port and I had to insert the USB stick at a specific time.
I'm running PLoP on a Lenovo T60, where the USB boot option is disabled in the BIOS (and I don't have admin rights to change it). To make it work, I can't have the USB stick plugged in from the beginning (i.e. when the BIOS startup screen is displayed), but I must wait until immediately before PLoP starts loading from the CD. If I plug it in after PLoP has loaded, it won't work either.
Also, my laptop has three USB ports - one on the left hand side and two on the right hand side. To be able to boot from the USB stick, I have to plug it in to the lower one on the right hand side (not sure wich host no. that is) or else it will not work.
BTW, I'm running the latest test version of PLoP and haven't had the time to try this with the latest "official" version. Hope this helps someone in either booting from a USB or Elmar in finding any problems with PLoP.
Finally, thanks for PLoP - it works great!
