I'll take this opportunity to slip out of my lurker status since I just recently put clips on my fixie.

Platforms are just fine for free wheel single speeds - I did it for about a year and a half.

I retained them when I went fixed recently but, after riding my brother's fixie, I noticed that the clips allow two things that the platforms did not:

1. With clips, you can pull up on your "down stroke" which helps control your speed, particularly on hills.
2. Skidding ... I've yet to do it out of necessity but it sure is fun.

After using a pair of clips that were lying around the house but did not really fit my pedals, I got a pair of MKS track platforms with the matching clips for around $30 new. I usually shy away from product endorsement but these things rule. I had always had trouble clipping in before but these have a nice metal tongue thingy that you can use to flip them around. I clipped in with out even meaning to the first time I rode them. It's like they are hungry for feet or something.

I hope that helps.