I was also looking for a solution to check if variations are selected. I had altered the products page and didn't notice I'd changed the class of the "add to cart" button.
As Mikeagg (sp?) said in another thread:
If anyone has this problem in the future, you need to make sure that you have your add to cart button and variation drop downs within a div tag with one of the follow classes - productcol, textcol, product_grid_item, wpsc-add-to-cart-button
You also need to make sure that your add to cart button has the class wpsc_buy_button and that your variations are held within a div with the class wpsc_select_variation
As soon as i had these classes, the validation wobbled back into life :D
Incidentally, I haven't updated to the latest WP-eCommerce since I tried that and it crashed my whole site. I reverted back to 188.8.131.52 as it works nicely.