I think most people on the testnet site were trading with the $10k allowances they got given, and the 50BTC that you can pretty trivially solo-mine on testnet. So no, I don't think anyone ever actually tried trading tiny fractions of a Bitcoin on it.

I was mistaken also, it's not rounded to one decimal place - it's rounded to the nearest 0.05.  So that means that with 0.78 BTC in my account, I'm only actually able to do anything with 0.75 BTC.

MtGox was frustrating like this too - I have some tiny fractions of a US penny and some fractions of a Bitcoin in there that I can likely never do anything with.