Blimey! This just shows my lack of knowledge I suppose but I do exactly what Oggy does (as I'm currently trying to get back into riding clip-less on my MTB again) and stuck my old spd pedals on the roadie and put some new'uns on the MTB, both Shimano, so I can get away with using only one pair of shoes There's a credit crunch you know
They're a road pedal that's built to take the mtb cleat and work an absolute treat with my Northwave boots and Specialized BG shoes. I've been using the same set for the last five years or so and they've handled sportives, multi-day trips and the everyday commute with ease. Cracking bit of kit.