I don't understand the need for fluid grid layouts when you can use CSS media queries to find out what device/resolution is accessing your site and serve it the appropriate styling that way?
I only want to cater for mobile phones and desktops (not tablets, I consider them to be same as desktop). So I simply create a media query that looks for a maximum width resolution of 480px and serve it a mobile version of my CSS file. Everything else gets the full desktop CSS file. I can't see why fluid grid layouts would help in anyway extra unless I'm missing something major here?
Also on a side note if you do present a mobile device with a cut-down CSS version, how can you offer them a 'desktop site' link to get the full site on their device? That is, how would you get around the CSS media query for mobile phones if the user doesn't want the mobile version?