There are efforts with the Debian-based distros to make a universal Debian system. It's called the "Debian Common Core Alliance". So if any Linux distribution goes mainstream, it will be a Debian-based distro. Right now Xandros and Ubuntu (and all Ubuntu derivatives such as Kubuntu) have the most buzz of the Debian-based distros. I predict Linspire will eventually be lost in the shuffle and fall so far behind that they will probably be sold to Xandros.
The DCC is a step in the right direction of establishing a common base of compatibility, but developers will need to actually invest in development for that common base to a degree where the resulting user experience is consistent across applications.