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:

1. Open Notepad and type one TAB. Select it and copy it.
2. Go to OneNote and click Tools > Auto-correct Options...
3. In the dialog, put your desired sequence of characters (e.g. \t, or \tab, or TAB, or <t>, or whatever you like) into the "Replace:" field.
4. Paste the TAB that you copied from Notepad into the "With:" filed. Click Add.

Now, any \t followed by a SPACE will be replaced by a regular TAB.