I've got a wedding coming up and would like to know about the GS400's 'soft skin' feature. Does it make the skin look softer and more radiant and sort of blend away any blemishes? would you use it in a wedding montage?

Yes, it's supposed to do exactly what you say.
How usefull is it you can decide by taking some test shots.

Jeremy, an indoor or outdoor wedding?

It's an indoor wedding. My concerns are that any advantages with soft skin would be that the skin tones would come out with a bit of noise from the blurring and subsequent encoding to mpg2.

I would be more concerned with light levels during the cermony. Have you tried a test run? Your footage may be so dark that the skin tone feature has little to no effect.

I say this because I shot an indoor wedding last year with a PDX10 (also 1/4.7" 3CCD) and the footage was useless and I was lucky to have a VX2000 (1/3" 3CCD) as the primary cam. The bride's mother would not let me raise the lighting by even one candle. :)

Actually, the use of 'soft skin' will be used for a vignette. The actual wedding will be shot with the camera on it's normal function.

It might be better to use a mist or softening filter if you like that look...or you can do it in post. I'd shoot with all the resolution I could get, then soften it in post. That way, if you don't like the look, you can go back to the full resolution image. I also agree with Bogdan...shoot some first with soft skin before using it for real.

Definitely agree with doing it in post, especially if shooting in a low light situation. Many more choices re getting the look you want when you start off with a decent image and good software. (Says she who screwed up one scene by misadjusting the filter.)