For an upcoming article I'm writing for Catalog Success on alternative payment methods, I interviewed Vince Talbert, VP of Marketing for I4 Commerce about BillMeLater. I enjoyed speaking to Vince -- BillMeLater is a solid product, and Vince was friendly and insightful.
Contrasting BillMeLater with PayPal and GoogleCheckout, Vince noted that PayPal and GoogleCheckout when marketing their wallets to consumers stress the benefits of keeping private information (credit card, email, etc) from the merchant.
In contrast, Vince argued BillMeLater creates reciprocal trust from the shopper to the retailer because the retailer is extending credit on the spot. He invoked the image of an old time grocer, who perhaps let your grandparents "buy today and pay later."
Aside from the nuts and bolts of implementation, and beyond the financials of marketing benefit and processing fees, it is interesting to see how these three payment products position themselves with respect to the customer / merchant trust relationship.