I just received some samples of the SeaDek. The stuff seems like it would be pretty nice for outside the head, might even consider it for in the shower. The material is about 1/4" thick, made from soft squishy closed-cell foam. There are little raised dots on the surface that should provide good traction when wet, but still be comfortable for bare feet. It has an adhesive backing which seems like it would stick to most surfaces. It looks easy to keep clean and should be warm to step on.
Despite what the product literature says, I'm not sure I'd use this material outside. It doesn't look like it would hold up all that well to UV. It also doesn't look that "tough." I think it would be prone to cuts if something dropped on it. And I think the raised nubs would wear off pretty quickly in high traffic areas. It should have nice thermal and sound deadening properties.
So, I think I'll go with the SeaDek for below. Now to pick a color ....