Only if they refer to something non-generic in their domain of application. Microsoft nearly lost the "Windows" trademark with the Lindows lawsuit, because a window was a generic term for the kind of UI that Windows does (when they realised that they might lose the trademark, they decided to settle out of court with Lindows instead). You can't sell produce under the name Apple, but you can sell computers under that name. "Dropbox" is a more generic term than "Infinite", so I can see how it could run into trouble if someone came up with a similar file sharing service and also called it a drop box.