Okay Alpha_Omega, I took a good hard look at my Gamecube last night, and I was able to a few very faint cracks under the surface of the plastic. They were all in the places you described yours as being. Those are nothing to worry about. When plastic hardens, there are places where the stress on the casing is greater and there will undoubtedly be small, harmless cracks because of it. They will not spread, they will cause no harm, they are nothing to worry about.

I thought you were talking about plainly visible cracks on the very surface. That would be abnormal.