In reply to TobyA:
Hey Toby, congrats on getting into cyclocross. It's my favorite discipline of bike racing, bar none. I did my first race back in 2003 and have gone through a host of different pedals, so I'll give you my take, for what it's worth. For starters, you definitely want SPD type pedals... no flats, and no add-on cages. Cages cause the pedal to flip upside down when your foot's not attached(weight, center of gravity) and the upside down cages get caught in the dirt after dismount (causing problems), and after remount before you've flipped them over (causing big problems, either broken pedal or a wreck). Definitely no cages. As for what brands/models of SPD style pedals:
Crank Brothers Candy: my favorite pedal. Basically an Eggbeater pedal but with a plastic platform to give you a base to push against until you've clipped in. Eggbeater styles shed mud better than any other, and the cleat-clip interface is really easy to force to work when clay mud has jammed into the cleat. Lastly, the eggbeater style is the easiest of all pedals to clip into and out of.
Crank Brothers Eggbeaters: All the benefits as above, but without the additional plastic platform, which is sorely missed as it's a pain without them right after a remount if you haven't gotten your clip-in technique dialed in (as in, the second your shoe hits the pedal it clips in).
Time ATAC XC: 2nd favorite pedal. Not as much easy float (foot rotation) as the eggbeater style, but in exchange they have a really solid attachment and click in and out. They clean mud reasonably well. I liked them a lot, and prefer them on my mountain bike. But for cyclocross where there is so much in and out of the pedals, and running in mud, the Candy eggbeater style just excels. But I like the ATACs better than most of the Shimano ones I've used.
Shimano or other: I only have experience with the XT and XTR level pedals (both expensive). Over the years I found the lower end models didn't deal with mud in the cleats as well as the upper end models (although that may have changed these days). I'd say here in the states the Candy/Eggbeaters are the most popular, then shimano, then the ATAC (better in mud in my opinion than Shimano).
Hope that helps. Remember to remeasure your saddle height once using the new pedal setup. Also, for first time 'crossers it's not unusual to drop your saddle height 2-3cm from your optimal road saddle height (at least for the first couple races). It makes a big difference in being able to remount cleanly during those final laps.
Cheers, and good luck.