Xcode 4.1 installation not customizable? Missing Installation Type Step?
You can control or right click to show package contents and run only select portions of Xcode 4.1.

The program Pacifist will let you look into the packages to see what each contains.

The installer has changed to remove the options - it's all or nothing. If space is tight on your boot drive, you will have to either install to another volume and move parts back or temporarily free up enough space to install and clean out what you don't want to keep.