You cannot install Steam games anywhere other than the Steam folder, which limits them to being installed on just one drive. However, there is one workaround if you want to try it:
I believe Windows vista and 7 support symbolic links, so you may be able to work something out using those. A symbolic link is basically a shortcut that pretends to be a directory - so any programs will see it as a normal directory, but when they open it, they will be taken to the folder it points to instead, which could be anywhere on the computer. Google for detailed information on how to create them, or just copy my example below.
So, what you could do is install Steam and your games on your normal HD, then cut and paste the folders of the games you want onto your SSD, and create a symbolic link in the Steam folder for each one, with the same name as the game's folder, and pointing to the new location.
So, say for example, you installed Steam on drive C:, but wanted to put Audiosurf on drive D:. You would first install it with Steam as normal, putting it on drive C:, then cut and paste the audiosurf folder from "C:\Program Files\Steam\steamapps\common" to "D:\Games". Next, you would open up a command prompt with elevation (ie. run cmd.exe as an administrator), and run the command "mklink /D C:\Program Files\Steam\steamapps\common\audiosurf D:\Games\audiosurf". Now, when Steam tries to access the audiosurf directory, it will be automatically be redirected to the folder on your SSD instead, completely transparently.
Note that if you choose to uninstall the game through Steam, it will probably just delete the link instead of the game, so you'll have to do it manually. This technique also isn't possible in Windows XP, to my knowledge (and I've never tried it in Vista, though it should work the same).
Last edited by The_Terminator; 04-24-2011 at 03:23 PM.