Currently finishing off a new plugin for Virtuemart 2.0 that creates a length x width calculator.
This plugin will allow the administrator to specify an area value for a product and then allow the user to purchase on area value. The length times width calulator asks the user in the product for a length and a width meaurement. It then multiplies these together and multiplies by the cost per (m/inch/ft/yard) and inserts the total cost into the basket of each item selected.
This will include some funky jquery coding and a bit of PHP development. We're sure other Virtuemart Developers and Joomla/Virtuemart website owners will find it useful.