I had the same problem when I swithed my older Dell motherboard to a new case. I found an adaptor that worked perfectly from Centrix at
http://www.centrix-intl.com/
(search for Dell under misc., it's called a DellCv)
here's their picture of it:
http://www.centrix-intl.com/DELL.jpg
In the meantime, the pin configuration of the original power switch to my Dell motherboard was a single plug but my new tower has separate pins for the power swith, reset button, Power LED, HDD LED and Speaker. Does anyone know where I can find a map of the pins so I know where to stick each plug?