Why does my keyboard switch language in most input fields on the web?
I've had weird input-switching behavior before when the keyboard shortcut to switch input sources was the same as something else I was using, like Quicksilver.  Try disabling or changing the relevant shortcuts in System Preferences (under Keyboard &amp; Text Input).

UPDATE: Detailed instructions:


Open System Preferences
Open the Keyboard preference (under Hardware)
Select Keyboard and Text on the left
Find "Select the previous input source" in the box on the right and uncheck it.  Its shortcut was probably set to Command-Space.