I don't think the warning in Finder can be disabled. However, if you frequently do mass extension renaming, Automator can handle this task easily, without generating warnings for each file.
For those that don't know how to use the Terminal to rename files:
The command you need is called mv. Here's how it's used:
Code:
mv source-path destination-path
The Terminal is nice, and lets you drag and drop files and directories on it to copy their location to the prompt. I highly recommend doing this. For example:
Open Terminal
Type "mv " (make sure you include the space after the command)
Drag and drop the source file or directory onto the Terminal window
Type a space
If renaming a file, drop the file again, and edit the dropped text to what you want