In 10.6, the behaviour of input language switcher (Command-Space) has changed. Before, it would change the language immediately upon keypress. Now it waits until you release Space, and if you don't rekesae it for some time, the system will display a HUD with a list of input languages (see this hint about that behavior).
The problem is that not only this is absolutely useless, it's also irritating. First of all, sometimes the system switches to a language you didn't want to switch to (those who have encountered this problem will understand). Second, this behavior conflicts with Photoshop's Command-Space modifier, which is used to temporarily activate the Zoom tool: when you try to select the portion of image to zoom in on, the stupid HUD appears and obstructs the image. Third, it feels sluggish and unreliable.
The solution came to my mind when I found out that old input switcher behaviour was triggered by using the Shift-Command-Space keyboard shortcut.
I was able to make the switcher work as well as it did in 10.5 with a help of Spark, which is, by the way, a great tool for assigning keyboard shortcuts to various things. Here's how I did it:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091117105330475