Although it's true that the spec mentions that all payments after 2013-08-31 will be considered invalid it does not tell us which time to use for this calculation. If we want to be precise then yes 255361 would be the valid answer. However people sending coins on 23:48 did so thinking they send the coins before 2013-08-31, which they indeed did. It seems more fair to use block 255362 since this would include all transactions _send_ before 2013-09-01. (and yes; perhaps 58 seconds of transactions that were sent after)
For now, this really doesn't have much effect. But in about 5 years - this loss of transaction is going to cost someone about 13 million dollars. Better take a moment to get it right. I'd include them. Leaving them out exposes you to a claim later by a fast talking lawyer who will argue ambiguities in the spec should be enforced
against the author rather than the investor. He'll win with that line of reason. The writer of the document had the ability to set it correctly - and that failure results in the liability shifting towards him.
Now, no lawyer will touch this case. When it is worth $13 million there will be a long line of shark attorneys at the courthouse filing paper.
I know it is a stupid way to settle the question, but it is worth considering.