Every time I type "tm" in Safari, or an app that uses a Webkit control, it changes it to ™. How do I stop this from occuring?
By default, OS X has system-wide symbol and text substitution. You can disable this one substitution, all substitutions or even modify the TM trigger so you are less likely to accidentally trigger a ™ inadvertently.
System Preferences > Language & Text > Text pane/tab