Renaming, deleting or hiding a build type is not directly available from the Team Explorer or from any menu or tool bar. Essentially a Build Type's name is same as the name of the sub folder in which the build type files are checked in. So if there is a team project MyTeamProject with a Build Type MyBuildType this means there is a source control folder on the server with the server path

$/MyTeamProject/TeamBuildTypes/MyBuildType

On Beta 2 bits the path will be $/MyTeamProject/TeamBuildConfigurations/MyBuildType

So renaming means renaming the folder named MyBuildType. Deleting would mean deleting this folder. If you want to hide this Build Type from coming up in the Team Explorer you need to rename the file TeamBuild.proj inside this folder to some other name ($/MyTeamProject/TeamBuildTypes/MyBuildType/TeamBuild.proj). The steps are as follows

Adding Workspace Mapping

Edit workspace dialog First you need to have a workspace that maps the TeamBuildTypes folder (or TeamBuildConfiguration on Beta2 bits). To do this go through the menu File -> SourceControl -> Workspaces... Select a workspace and click edit and then add the mapping similar to the one shown in the screen shot. Rest is more easily done from the command prompt.

Open a Visual Studio 2005 command prompt. In that cd into the local folder that maps the TeamBuildTypes folder. Run the following command to get the latest files
tf get * /r.

Hiding a Build Type

cd into the MyBuildType folder and run the following
tf rename TeamBuild.proj foo.proj
tf checkin

This should bring up the checkin window. Click on the Checkin button. Now if you refresh the Team Explorer the build type should go away from under the Team Build node.

Rename Build Type

To rename the BuildType go to the folder above and run the following 
tf rename MyBuildType CoolBuildType
tf checkin

On refreshing the Team Explorer a new Build Type named CoolBuildType should come.

Deleting a Build Type

To deleting a build type run
tf delete MyBuildType
tf checkin