When deploying a web application across your organizations hosting environments you need to make sure you make the correct modifications to your web.config on each environment. Currently with Visual Studio 2008 this means hand modifying each web.config to make connection strings point to the right database servers, changing the debug compilation attribute to false etc. Doing this for each deployment can be a major pain and can allow bugs to slip into your environments, for instance you may forget to modify your connection string and have your production application pointing to your staging server or have your application underperform because you have left debug compilation on.
Thankfully the Visual Studio team has come up with a great feature to be included in the next release of the IDE that allows you to keep a copy of transformations for each web.config. Your transformations will get applied to your main web.config when the application is deployed to each environment. This should greatly improve the deployment experience and help keep deployment errors down to a minimum.
Channel9 have a video all about it so head over to there and check it out.