I generated block 100283 and collected a sub bitcent transaction fee.
I get an "Insufficient funds" error when trying to send my exact balance to another address.
It seems that there is either :

• wrong behaviour (should send the amount), or
• wrong error message, (the balance is ok but the client forbids me to send it)

Block : 000000000003d014210dcffd602715545bce976f0f257d99279c83c2778e7150

Here's the trace

david@box:~$ bitcoin getinfo
{
    "version" : 31900,
    "balance" : 50.00638641,
    "blocks" : 100760,
    "connections" : 48,
    "proxy" : "",
    "generate" : true,
    "genproclimit" : -1,
    "difficulty" : 14484.16236123,
    "hashespersec" : 9273499,
    "testnet" : false,
    "keypoololdest" : 1291071649,
    "paytxfee" : 0.00000000,
    "errors" : ""
}
david@box:~$ bitcoin sendtoaddress 12DidCRRT9fqmSBReRNN8KjCktYGTVU1Jj 50.00638641
error: {"code":-4,"message":"Insufficient funds"}