I consider this more of a feature rather than a problem. For instance, I received a Dell Studio 15 from a friend that would not power on with the power button, seeing as the cord was internally ripped. It did however, boot with the media button once the battery was charged. Usually it was a combination of the network button and media button however.
Anyway, I'm not sure what started the media button power on, but if the power button does not power it on all I can think of is that the laptop recognizes the power button is malfunctioning and somehow allows you to power on with the media button.
I also know there is no setting in the BIOS that turns this on, so I also find it a little strange. You might want to try contacting [http://support.dell.com/support/topics/global.aspx/support/dellcare/contact_technical_support?c=us&cs=19&l=en&s=dhs&~ck=anavml|Dell Support] to see if they have a solution. Best of luck, if you find out what the issue is, please let us know!