guys...it's really simple. It's just like 99% of all the other credit lines / credit cards out there.
BillMeLater works the exact same way, in fact they are the same company.
All transactions are treated individually.
A single order > $250 gets 0% interest for 6 months
A single order > $500 gets 0% interest for 12 months
If you buy a TV today for $1500, you pay zero interest on said purchase as long as it is paid off on/before 12/31/2011.
If you then buy a video card for $350 on January 15th, you pay zero interest on said purchase as long as it is paid off on/before 7/31/2011.
On 8/1/2011, if you didn't pay off the video card, well you just got hit with the differed interest over the last 6 months, and will now pay monthly interest charges on it. The original $1500 purchase is not effected until it expires months later.
When you make a payment, you can tell them to apply it to a specific purchase, rather than spreading it across multiples (default).
So in short, just plan accordingly and pay off the items before they expire, paying interest on these accounts is a poor decision.