Not for the old ones (mine are ?3 years old), I left them on all day once luckily nothing happened but there was a distinct 'burning' smell when I returned home!!
Nope, as alijangra said before, unfortunately they don't. :o(
I, too, have left mine on all day, on my bed! Luckily they were on the heat mat I bought for them, but they still managed to scorch my bed linen through it.
there was a documentarie on them (watchdog) that claims that they even when plugged in and switched off can cause fires so please switch completly off they had some bad press so be careful ladies and gents ! x