Putting aside numerous other issues, bitcoin.org states "The total eventual circulation will be 21 million bitcoins".
[EDIT: DanI-S pointed out that bitcoins can already be subdivided. So this was switched from "there are too few bitcoins to support even American e-commerce" to "bitcoins are not valuable enough to support even American e-commerce". The number of fractional bitcoins is still an issue, but less immediately.]
At the current ~$0.70 / bitcoin, this means that every American will be able to have ~$0.05 in his or her electronic wallet, once all bitcoins are generated. Assuming that the rest of the world does not participate at all and that bitcoins are evenly distributed.
Sure, you could imagine an instant dollar-to-bitcoin-to-dollar conversion at the point of payment. Or you could imagine a bitcoin2.org that generates more coins. Or you could hope for a massive surge in the value of the bitcoin.
I'd put my money on Paypal sticking around, though.