Ok, after a long research I narrow the search down to these two cases:
1. The Quattro-T4 Glass touch-thru case by IvySkin
Description from IvySkin's website:
Announcing the 2.0 version of the most award winning iPhone case! We are always looking for new and exciting ways to improve the quality of our products.
Curious about whats new in Quattro-T4? Of the dozens of new upgrades, here are some we think youll especially like:
1. Improved Glass Touch-Thru screen
2. New UV coated Polysilicon shell that will not fade and it's impossible to destruct.
3. Upgraded the worlds first Polysilicon tactile solution to enhance the tactility for the button you use the most - The Home Button.
4. The fit. It is now easier to insert and remove the iPhone from the case.
The Quattro T4 features a patent pending Xylo Glass ® technology for the ultimate in Touch-Thru design. Authentic glass ensures that you will not lose any sensitivity while using this case, guaranteed. Designed to fit the new iPhone 3GS and current 3G iPhones.
Images:
Link:
http://www.ivyskin.com/quattro-t4.html
2. The Defender clear membrane case by Otterbox
Description from Otterbox's website:
The OtterBox Defender Series for iPhone 3G / 3GS offers stylish and sophisticated protection in 3 layers. First, a thin, clear membrane covers your Touch Screen to help prevent scratches and dings. Second is a hard, strong Polycarbonate skeleton that surrounds your device. Wrapped around all of this is a Silicone skin that absorbs bump and shock.
Images:
Link:
http://www.otterbox.com/iphone-cases/iphone-3g-3gs-cases/iphone-3g-3gs-defender-case/
My take:
I like the OtterBox double layer but it looks to me that it would be too bulky. I like the IvySkin slim design and all the things they compacted into this case but I need to know what other people think...
Let me know!
1. The Quattro-T4 Glass touch-thru case by IvySkin
Description from IvySkin's website:
Announcing the 2.0 version of the most award winning iPhone case! We are always looking for new and exciting ways to improve the quality of our products.
Curious about whats new in Quattro-T4? Of the dozens of new upgrades, here are some we think youll especially like:
1. Improved Glass Touch-Thru screen
2. New UV coated Polysilicon shell that will not fade and it's impossible to destruct.
3. Upgraded the worlds first Polysilicon tactile solution to enhance the tactility for the button you use the most - The Home Button.
4. The fit. It is now easier to insert and remove the iPhone from the case.
The Quattro T4 features a patent pending Xylo Glass ® technology for the ultimate in Touch-Thru design. Authentic glass ensures that you will not lose any sensitivity while using this case, guaranteed. Designed to fit the new iPhone 3GS and current 3G iPhones.
Images:

Link:
http://www.ivyskin.com/quattro-t4.html
2. The Defender clear membrane case by Otterbox
Description from Otterbox's website:
The OtterBox Defender Series for iPhone 3G / 3GS offers stylish and sophisticated protection in 3 layers. First, a thin, clear membrane covers your Touch Screen to help prevent scratches and dings. Second is a hard, strong Polycarbonate skeleton that surrounds your device. Wrapped around all of this is a Silicone skin that absorbs bump and shock.
Images:

Link:
http://www.otterbox.com/iphone-cases/iphone-3g-3gs-cases/iphone-3g-3gs-defender-case/
My take:
I like the OtterBox double layer but it looks to me that it would be too bulky. I like the IvySkin slim design and all the things they compacted into this case but I need to know what other people think...
Let me know!