We have three 8gb original iphones, still holding up great, battery life seems ok, but sim cards are removed, now used as touch for kids since we replaced the phones with 3gs 2 years ago.
Cases for them are hard to find. We have silicone on the originals now, they are cheap, and protect the touch/phone if dropped.
The incase slider is awesome, I used one on original, and also on 3gs for awhile. Yes, it will scratch back, so will silicone, it's not the case, it's the dirt that gets between case and back.
Michael