They are very easy to take off and leave no mess or anything!
Think of it like a plastic post-it note.
I had a some decals from decalgirl for my PSP (again to prevent scratching) and when I sold it to get my Wii i removed the decal and there was no marks or anything - it was quite litterally in mint condition!
The first thing I did when I got my Wii is order the decals and put the empty box in the loft. In a couple of years time when the Wii 2 comes out (as no doubt it will) I will peal off the decals and my Wii will be scratch free so therefore have a slightly better resale value.
You can get decals from eBay etc. but personally I would only get them from decalgirl.com and wait a couple of weeks for them to arrive from the USA as they are infamous for top quality decals.
