The majority of those scratch removes are simply overpriced mild abrasives.
I strongly recommend trying Brasso first before buying one of those.
See this:
http://todd.dailey.info/archives/200...can-of-brasso/
I have used brasso on my ipod and mobile phone (k800i had a scratch on the screen) and after a while, it works perfectly. Took quite a few re-applications and rubbing get the the deep scratches out, but worked perfect in the end.
After cleaning my ipod, I applied the invisibleSHIELD. I can confirm 100% that when I removed the shield, it left no residue marks at all and came off cleanly.
Therefore, I am guessing that good quality phone screen protectors shouldnt leave any residue marks either.