Have a look at the 'Build Products' section of my post about Xcode 4 here
. This will be the default location of the build products. You can also find the location of the product by expanding the "Products" group in the source tree (at the bottom left), ctrl+clicking on the product and clicking "Show in Finder".
Additionally, Jules has a added a script which runs after the link stage to copy the product into the user's plug-in directory at "~/Library/Audio/Plug-Ins/Components" that should allow hosts to find it.