I got the Lego Club magazine a few days ago and Hero Recon is where you can use LDD to make your own Hero Factory robot, and it's comming in April.
Just throwing that out there incase anyone didn't know.
I have LDD now and started building a tiny railcar. But I have no rails. :C
If you're going to buy it, well, try adapting the wheels so they have a 1 stud wide gap and make your own rails.
If you're not going to buy it, go to Program Files > Lego Company > Lego Digital Designer and open prefrences.inf and enter the following:
themeAccess=legoUnivurse
This will enable another extra catagory that has nearly every Lego part in existance and in every color.
It's a shame that when they updated LDD to version 4 it required a better graphics card so I have to use it on my sister's new computer. :c
AND THEY ADDED A LOT OF COOL NEW HEADGEAR TOO IN legoUniverse MODE :C