As you may know, pressing TAB in the middle of a line in OneNote 2007 creates a table. There is no way to turn off this behavior, but if you want to create a regular TAB instead, you can use this workaround:
Create an Auto-correct rule that turns your chosen character sequence (e.g. "\t") into a regular TAB:
Now, any \t followed by a SPACE will be replaced by a regular TAB.