The negative amount is the transaction fees earned by the miner for generating the block.
A Bitcoin transaction is made of input amounts and output amounts. To pay a transaction fee to get your transaction included in a block in a timely fashion, your Bitcoin client crafts a transaction with a larger input than output. the extra money that doesn't come out of transactions is the fee. The miner gets to keep the transaction fees of all transactions they include in a block.
The previous posters are misinterpreting the information shown about the transaction where the miner gets paid 50BTC reward + fees earned. The input is 50BTC, but the output is 50BTC plus the fees earned, so the block explorers show a negative amount when calculating the "fee" for that transaction.
No, this isn't what this is, there really are serious display errors in blockchain.info. Neither blockexplorer nor blockchain.info have a habit of displaying tx fees paid to the miner as "negative tx fees". http://blockchain.info/block/0000000000000765555944646925edf0449cb6d33c7d8ecf335b01aec1d94ace
is a summary of block 168425 which claims that the total transaction fees paid is negative, which is impossible. And if you look at the transactions in the block, many are displayed as having no input.
I wonder what kind of bug caused this, things are pretty messed up.