Hmmm... I got an error message when trying to send 5 BTC with more than 1252 BTC fee. Interestingly, when I decreased the 5 BTC by 1, I could add 1 BTC to the fee. I forgot to write down the error message, but I can try again in 120 blocks when I get back the 1257 BTC. (This was in testnet, of course.)
Not that it really matters (who the hell sends transactions with >1000BTC fees?), but I thought it was worth sharing.