I'm sceptical more of the fact it's addressed to Al-Zarqhawi - something of a nebulous figure at best and one who many believe is simply used as a convenient public relations 'figurehead' by the coalition for the public to easily identify with.
A bit of googling turns up the official US government
press release. There's also a
BBC article with nothing to add. They do point out however that the last letter said to have been intercepted (in 2004) never had its "authenticity confirmed beyond doubt".