From the thread here:
http://bitcointalk.org/index.php?topic=287.0main.h: // To limit dust spam, require a 0.01 fee if any output is less than 0.01
The general idea seems to be that once a BTC is worth more they can change this value.
lachesis points out that:
I think a 0.01 BTC minimum send is fine for now. At the current exchange rate, that's 8/1000 of a penny USD, or 80 millionths of a dollar. Any micropayment smaller than that would not be made one at a time....
I'm not aware of any other transactions fees but, I am not the expert by any means...