As you can tell from the amount of posts today, Arno and I have been checking out the new FSX SDK this weekend (that is only in the Deluxe version btw). One thing I have known about for a while is that there are some plugins you can enable to help with design in FSX.
There is a Object placement tool, an Effects tool and a Traffic Toolbox tool.
To enable these, open the DLL.XML file in the documents and settingsyournameapplication datamicrosoftfsx folder and change the <Disabled>True</Disabled> to <Disabled>False</Disabled> in each case. Then you will have a new Tools menu when you load FSX.
Arno almost went crazy when he started playing around with the Object Placement / Mission Builder tool. We have already found some limitations, which we may be able to work around, but I am sure he will post something soon on his blog.