We also need this feature. In my opinion, you should be able to generate a SKU automatically if desired, which would fall sequentially after the last issued SKU or overtype the SKU to create your own. Attempts to create two separate products with the same SKU should not be allowed or at least an "are you sure" message should pop up (though I can't figure why someone would want two products with the same SKU)
Keep us posted!
We would like to disallow duplicate skus and handles (rather than shopify automatically appending -1,-2,etc) through the API as well.
Before inserting a product via the API, we check to see if it exists by the handle
I suspect due to some recent indexing problems, a recently inserted product is not being returned through the API as it's not showing up in the index & we then insert it later and end up with either a flat out duplicate handle and sku (which shouldn't be possible), or we end up with a two items that are essentially duplicates with handle of 12345 and sku of 12345 and the second record has a handle of 12345-1 and sku of 12345.
An option to enforce uniqueness of handles and skus on api add/update might solve these problems.