I know I've already added my ha'p'orth (remember them old half pennies - now you really felt you had money with a few of those in your sweaty palm!) but I feel I must add some more as the thread seems to have moved from "should we move the festival" to more of a security issue. I see the solution as:
1). Don't take anything really valuable.
2). If you have to bring stuff (credit cards/mobile/iPod etc) carry it with you at all times. Don't ever leave anything unattended outside the main field.You might think "whoever would nick an old sleeping bag ...." some people nick anything.
3). When you stake out your "viewing plot" speak nicely to the people sat around you - they'll be more than happy to keep an eye on your chairs/coldbox/wet weather gear as you of course will reciprocate - but carry your valuables with you when you go for a pee/drink/chow. As far as I'm aware there's never been any muggings! if you can see your valuables they're a lot less likely to be pinched.
4). Take a decent padlock for your tent and don't leave anything outside it, even (especially) when you're in it asleep. Most thefts are caused by opportunists - make it difficult for them.
5). Keep your bike panniers, caravan, car etc locked. Chain bicycles together.
6). Remember that the overwhelmingly vast majority of your Cropredy Companions are lovely, honest people but it's worth taking these few measures to thwart the few bad pennies. Make it difficult for them and they'll soon move on to where it's easier.
Remember - even in the Good Old Days when we all left our front doors unlocked, there was still robbery & theft - the thing is to make it as hard as possible for them.
Oh, and if you catch anyone at it, I like the previously-mentioned idea about re-enacting the battle of Cropredy!