We get this question a lot and there's more that plays into signal strength than the case. Apple has upgraded their whole antenna configuration and should help with any potential signal issues over prior versions. >> http://www.macrumors.com/2011/10/04/iphone-4s-includes-significant-antenna-upgrades/
However, as I stated there's more to it than just the antenna. If your device doesn't have a case you can still see signal issues depending on how the device is held, where the towers are, etc. There are so many variables that effect this. The goal for this case was to reduce an potential signal issues the best we can. But with any case it's almost impossible to eliminate them because you are physically covering the antenna at it's source. If you see from this picture >>(http://kck.st/pDJgm2) we've added some EVA lining to help mitigate any potential problems.
Please also see update #6 http://www.kickstarter.com/projects/1791911961/i-case-iphone-4s-and-iphone-4-bumper-case/posts/130738
Jason & Chung