And just to give this a bump, I used the secure online account number thing last night for the first time. I get it now - you can generate as many secure numbers as you want, and once you've used one, it can't be used with another retailer - so even if someone does manage to get hold of the number via internet dodginess, there's not much they can do with it.
That was also my first time using ShopDiscover - it was great! I'd been Googling to try to find some possibly-long-discontinued foundation, and found that Drugstore.com carried it. I almost ordered, then thought 'wait... maybe one of those ShopDiscover retailers has it. Then discovered that Drugstore.com IS one of those ShopDiscover retailers. So I'm now getting 10% cashback for my troubles - woohoo!
Discover, me likey.