I just got an AVRISP MKII and found it didn't supply power to the target device. So I hacked mine.
I found lots of WAY too complicated and convoluted methods on the web... my hack only uses one part - a 1N4001 diode.
The diode needs to be bent into the proper shape to clear the board and to clear the ISP connector - other than that just solder it in and done!
Check the photos for more info (click on a photo for high res):Be sure the diode points the right direction! LOLDiode lead bent to clear board and ISP connectorSoldered to USB connector
Hope this helps someone.....