no:

No, the account balance will go negative, I just tried.
We're talking about accounts here.
They do go negative.

(as posted on SE)

I'm using the json API. I need to keep all the account balances positive: I call getbalance first, check if it has enough bitcoins to proceed, and then move or sendfrom.

This approach causes two problems.

Problem 1 is that this is not an atomic operation, so after the first getbalance the balance could actually be changed.

Problem 2 only happens with sendfrom: there might be a transaction fee, hence if the user sends his entire balance, he'll have a negative balance due to that.

Solution to Problem 2 would be to know in advance the transaction fee: I noticed (though I didn't try) I can set it with settxfee, but I found no call to get it, is there?

While I could implement some external lock mechanism to solve Problem 1, I'd rather use a bitcoind native method, though I get it does not exist, does it? Hacking the sendfrom function to have it check the balance before sending would be enough, I guess?

I have the same problem trying to access it via android, using Wind (IT) as provider.
No problem on my desktop, using Fastweb (IT) as provider.