I built and ran sipa's code and was just testing it out with the testbox [mike] created,

here is the result of listtransactions after making a few sends thru the gui
{
    "" : 3725.00000000,
    "Test" : 10.00000000,
    "Test2" : 5.00000000,
    "Your Address" : 10.00000000
}

Next I transferred 3726.0 to "Your Address"
{
    "" : -1.00000000,
    "Test" : 10.00000000,
    "Test2" : 5.00000000,
    "Your Address" : 3736.00000000
}

Should a balance be able to be negative?