I don't think that we need to have a dollar to playerworld currency exchange rate. Just do like other games and have a dollar to a global currency. "Global Gralats". These global gralats would not interfere with the normal playerworld currency, it would remain seperated.
For example Gunbound has in-gamed earned "gold" and money bought "cash". You can't buy in-game "gold" for money bought "cash", you can only buy "cash" items with it. Just an example.
I don't see how playerworlds could manipulate the system that way. They sell their items for global gralats and it's up to them what to charge for them. If they charge too much then nobody would buy the stuff so it's not a rip-off.
The only reason it would be better to have "global gralats" instead of just saying "pay with cash" is because not every country pays with cash. =P
I still say make it like 10 global gralats = $1 US. This way you could sell in game items for as little as 10 cents US but it still keeps an easy to remember transfer rate. You'd just tack on an extra 0 for items... like if you wanted a game item to cost $2 US you'd make it 20 global gralats... real simple. =P