Sorry Anand, missed that you already tried that. I got a chance to try out Microchip HID Bootloader v2.3 this morning on a 18f4550 using the Olimex PIC-USB-STK http://www.olimex.com/dev/pdf/PIC/PIC-USB-STK.pdf

and I got an LED blinky to work. For this chip, you have to hold down PORTB.4 to get it into bootloader mode. Then I can program and verify correctly. The hex I was using assumes a 12 mhz oscillator, and uses PLL to bring it up to 48.

Once my blinky program is loaded, I can hit the reset button, and my LED starts to blink.