How to remove device(s) from My Market Account? I know I can choose "Hidden in menus" and never see the devices again when I install a application. But I don't want my old mobile phones display in my account settings anymore. I have sold those phones already and never use them again!

In fact, the list look likes a cemetery in my mind now. ;(
As things currently stand, there is no way to disassociate a device from the ones shown in your Market Account. Keeping them hidden is the only way that Google currently offers.