Well as far as moving the steam files you can use Symbolic Links in Windows Vista/7 it will make a "Folder" that redirects to another folder making it look like the game is where it was (Uninstalling/patching will still work etc). This can be used by those with SSD's and 1 HDD to allow them to pick what programs are stored where. You can't move major files but you will be able to move applications to another drive after installing them either using "Safe Mode" or moving them when they are not running depending on the program.
EDIT: Oh and to get this to work you need to first move the application to the other drive then make a symbolic "Folder" that points to the moved folder with the same name as that folder.
Step 1 -> Move C:\Program Files(x86)\Steam\steamapps\common\portal 2\ to D:\Games\portal 2\
Step 2 -> Create Symbolic link C:\Program Files(x86)\Steam\steamapps\common\portal 2 Pointing to D:\Games\portal 2\
Wiki explanation-> http://en.wikipedia.org/wiki/NTFS_symbolic_link
GUI Utility -> https://code.google.com/p/symlinker/