I've got folding plastic pedals on my Tikit and Dahon D7. The only problem I have had is they get a bit slippery when wet so not ideal for winter use. I don't want to clip in on my folding bikes so just having a platform isn't an issue for me.

I'd like to find a grippy metal folding pedal with a flat platform. If anyone knows of one I'd love to hear about it.