I used to do a lot of Trademark work for a company that no longer exists because it was bought out by a billionaire yachtman recently.
Gresham--typically your TM protection only extends to the category (or similar category) which your TM was registered under (there are over 30,000 categories by the way). Take the TM "ORA" for example. ORA gets TM protection against all other marks in the same category that it's registered under (probably aquacultured fish). It also probably gets TM protection against all products that are in similar categories--probably anything having to do with fish. ORA does not get TM protection against products in completely different categories--such as geothermal power (see here: http://www.google.com/finance?client=ob&q=NYSE:ORA).
The more famous your TM is, the broader the protection you get. Take "McDonalds" for example (arguably the most famous TM in the world). It is so famous that it not only transcends the category in which it was filed under, it probably transcends all products in every category. You probably couldn't sell a new LED light fixture called McDonalds LEDs (or MackDawnoldz for that matter) without first getting a nasty cease and desist letter from the Hamburgler and a big fine/injunction if the matter made it to court.
As you can probably surmise, there are lots of arguments over what constitutes a "famous" mark. Like everything pertaining to legal stuff, that's why you hire the hot shot expensive lawyer to better make your case.
Well if you're still reading this, I guess that's your mini legal lesson of the day. BTW, there are a bunch of exceptions to this general rule so if your cousin's cousin's aunt's hairdresser has been selling McDonald's Soul Glow Hair Activator for years without any problems, there's probably a good reason why they are doing so. (OK, that example was just an excuse to link my favorite fake commercial of all time--see here: http://www.youtube.com/watch?v=ktl6L3ZwvL4)