It is a good marketing move, though who knows if it will help sell any Overdrives and the associated data subscriptions. It really is interesting to see the company not only suggest that iPad owners take advantage of one of its products, but Sprint went so far as to design an accessory for the iPad.
If you're already a Sprint customer and are looking to upgrade to a WiMax modem, I suppose it could make sense to take advantage of this offer. Or you could use a MiFi, or wait for the 3G version to arrive in the coming weeks, or, well, just use regular WiFi networks for your data connection.