This has been whistling round the web for the past few weeks (One rumour said that the cull was being planned to cut sheep and cattle numbers by half to enforce some Brussels dictat!). However, as far as I can gather from friends in MAFF and the MoD, it is complete cobblers - a rural myth, if you like, as opposed to an urban myth.
Certainly the MoD, in conjunction with MAFF, is now putting into effect a series of SOPs for dealing with any future outbreaks, including paper exercises in logistics, but I very much doubt that a mass, pre-emptive cull is planned.
That's not to say that the government didn't tell a pile of porkies when it said the FMD was licked, as the Yorkshire eruption demonstrates.
My own feeling over governmental matters is that cock-up rather than conspiracy is the order of the day. That's not to say that once something's been comprehensively cocked up there won't be a conspiracy to exculpate the guilty parties.