At this point, I don't think there's any technical reason that Linux on the desktop/laptop/netbook/smartbook wouldn't fly.
End users never see a command line in mainstream Linux distributions, anymore than they do with Windows 7 or Mac OS X (though power user still use them). Applications are available from a nice point and click GUI, very similar to the apps marketplace on smartphones, making software installs a snap.
Linux now has the best hardware support of any OS; hardware that doesn't "just work" is kind of rare these days. There's no driver installs, making the end user process even easier.
Since smartbooks will be the next big push, Linux will have an early advantage, as it did with netbooks.
All that said, unless more vendors start offering and promoting pre-installs, Linux on the desktop will not be mainstream. No one (expect we few) installs an OS. If viruses take over a machine, they take it to Geek Squad and pay $150 for e re-install. That's just the reality, just as most people don't do their own car maintenance anymore.
The single largest failure of Linux (which I use exclusively) is that it has no serious marketing. 99 out of 100 people I encounter have never even heard of it, after all these years. They know Android and Tivo and Google and Sharp and all the other brands that run Linux under the covers, but they have no idea what it is or that it's something they can use on a computer. I understand that the free distribution model doesn't lend itself to paying for marketing, but there isn't even a way to contribute towards advertising. The only thing Linux has is word of mouth.