One aspect of Linux distributions that we haven't done a good job of promoting is their use of package management and software repositories. For instance, we tend to refer to this using terms like "package management" and "software repositories" which don't mean anything to regular users. In the meantime, over the years people trying to use Linux often find that installing software is hard. This is mostly because they do not RTFM, and they try to follow the more difficult methods they learned for Windows and the Mac. It is also because the first things many users want to install are proprietary software like Flash, which can't use the standard methods for installing free software.
What the iPhone's App Store and Google's Android Market are going to do is get people used to the convenience of selecting and installing software from a central repository. Debian, Ubuntu, Fedora and other Linux distros have had the same thing for years, with one significant advantage -- by default, everything in their "store" is free. So this is a marketing opportunity for free software, if we can take advantage of it.