My guess is porn. Imagine the user- "Yes dear, I need to buy these Bitcoins to play this online game. It's pretty fun". Beats having all these odd payments on credit cards!
Well, yeah. There's porn. In fact one (at least) of the forum members runs three or four sites that take Bitcoin.
But the success of this currency is not going to depend on websites that take 1 or 2 or 5 Bitcoin at a time. It will depend on reliable transactions *beginning* at five figures, and going into six or seven.
And *that* is where a new Bitcoin client will have to focus: on 'splitting' Bitcoins into their intended - but never implemented - eight decimal places.
With only 21 million of them, I suspect we'll need the added functionality pretty soon now.
I see on the charts they're over a US Dollar...