Elaborate. What do you mean? If want just an aluminum back piece, there's half a dozen threads on here about that.
If you're talking about a rounded aluminum back, you're probably out of luck. I've seen a few people on here asking for rounded backs like the 3GS has (for example) and haven't seen any results
I got my custom MasElements sandblasted titanium back a few days ago. It's a really nice, well build backing. Build quality is excellent. It's looks like it came straight from Apple. The sandblasted titanium back looks just like the stainless steel antenna band. I ordered an apple logo sticker from Grafixpressions (not in pic) to finish the look.