Now that there are worldwide trademarks for the Arduino, they cannot sell a board as an 'arduino' if it is not. Technically they can sell it as a duemilanove as that is not a trademarked word.
The arduino team have done a bit of going after fake arduino boards but as they have been around for quite a while, it is difficult to stop them.
The new board should help slightly and it should be a bit longer before we start seeing fake Uno boards. I will be on the lookout and if I see any then we will hopefully get the people making them stopped.
If you do see a fake arduino on ebay then send ebay a message. They can then tell the seller to stop selling them of face a ban. Send ebay a link to the trademark info as well.
Mowcius