I have used the Canon PowerShot S90 for a couple weeks now and my initial impressions are very good. The image quality is better than other lower end point-and-shoot cameras I have used. The wide angle lens and large maximum aperture make this a great camera for the type of photography I enjoy. While I have not tried much low light shooting with the PowerShot S90, reviews I have seen all suggest this camera does better in higher ISO situations than most compact models.
As a former Panasonic LX-3 user, I have to admit the image quality is not as good as that of the LX-3. However, the Canon PowerShot S90 does offer slightly more zoom range in a more pocketable package.
Since I have not used a PowerShot G11, I cannot make any direct comparisons between the two cameras. Reviews I have read mostly favor the PowerShot G11 in terms of image quality and high ISO performance. The downside of the PowerShot G11 is the size and weight of the camera.
In the end, I think the PowerShot S90, PowerShot G11 and Panasonic LX-3 are all great advanced compact cameras. The decision to pick one over another will ultimately come down to the photographer's needs and preferences. Having held a PowerShot G11 and owned the LX-3 and PowerShotS90, my choice is the PowerShot S90. The combination of image quality, high ISO performance and portability make it perfect for my needs.