I would really like an aluminum bumper style case as well.
The biggest problem is that unless the case has a section of non conductive material somewhere around its periphery it will negatively impact the signal reception of the phone.
I saw this with my iPhone 4 and the Element Vapor case which I really liked the appearance but found that it made it unuseable at my place of work due to lack of reception.
The Element Vapor "Pro" was their answer to this where they inserted a piece of plastic polymer into the design to reduce this effect.
As we have seen already with the iPhone 5 and others experience here with all metal bumper style cases this seems to ring true again.
I would wait until someone comes out with a metal + polymer bumper unless you live/work/commute in a place where signal strength is not an issue.