Last time I checked, there was no discount for using bitcoin. For me personally, there was no incentive to use bitcoin. As a bitcoin enthusiast, however, I always check Overstock and Newegg first when making a purchase...and I use $US. Why should I spend BTC which I value when I have USD to dump on them? Why should I spend the exchange fees to purchase BTC and send it to them?
So the 1% direct sales is just another small part of a bigger picture that can only be estimated, much like all things decentralized, open source, and bitcoin.
I will argue that, over time this will likely change. Overstock is going to realize lower costs from accepting bitcoin then by accepting credit cards, and to encourage the use of bitcoin (and for them to have lower costs) they should give people an incentive to use bitcoin via a small (1-3%) discount