Mines been through 1 MOT with a nice shiny pipe instead of a DPF and it passed the emissions test by a country mile.
It will be going through another one in the next week or so and I'm not even slightly concerned about it not passing.
There's lots of rumour/speculation/internet gossip about the new 2012 MOT regs and checks for DPF's. I've read the new regs (unlike most it would seem) and as far as I can see there is no mention of visual inspection for DPFs, in fact there's no mention of them at all. There is plenty about Catalytic converters being in place if the car was produced with one but nothing on DPF's.