This information was correct as at June 2009, we hope to provide an update soon:
If you do not get acceptable mobile broadband coverage, you can return the product and cancel your contract within 14 days.
Note that online purchases are also covered by Distance Selling Regulations. Under these, you have a 7 day cooling off period. So, if you decided you didn't want the product, you would be entitled to return it and cancel your contract within 7 days.
If your modem is faulty, it will be replaced up to 28 days after the sale.
This returns policy also applies to Vodafone Small Business Mobile Broadband.