There's currently no way to do this. It has been suggested numerous times on Google's forums, but has never been acted on.
The best you can do for now is "hide" devices that you don't want to appear in the menus. You do this by logging into the Android Market website, then going to your account settings by clicking the gear icon and selecting "My Market Account" and then the "Settings" tab. On this page you should see a list of devices associated with your account. If you click the "Edit" button to the right of a device's name you can then select "Hidden in menus".