First, you can register whatever name you want that's available, in general there's very little restriction on that. Most major companies have already made sure to buy up domain names deriving from their brand.

Using the name of an open source package in your domain is fair game. But when you get into registered brand names, it can be problematic. The brand name does belong to the company, but people get away with it all the time. It happens because it takes a lot of time and money for companies to police the use of their brand and some don't bother. Some even get benefit out of it (for example, all of those World of Warcraft fan sites with WoW in their names). I would probably have no problem setting up the domain name, "mcdonaldsmakesgoodfries.com." But then again, maybe not. They could choose to ask me politely to remove it, or impolitely with a lawyer.

Interestingly enough, I may at some point have to move this site to a different URL if my publisher requests it. If that is the case, I will set up the webserver to send anyone coming to drupalfordummies.com to something like drupalfordummies.lynnbeighley.com.