Can I open files in TextEdit from the Terminal in Mac OS X? I frequently find that a Terminal command to open a file in TextEdit would be very handy. Does such a command exist on Mac OS X?
open -a TextEdit filename should do the trick.
The -a flag specifies any application you want, so it's applicable to any number of situations, including ones where TextEdit isn't the default editor.
Other relevant options

-t  opens in the default editor (i.e. if you use BBEdit, TextMate, etc.)
-e will open the file specifically in TextEdit