That's a great point. Never occurred to me to look at it like that, but you're right. Open source projects often fail to reach mass acceptance because they're designed by and for the people who work on the project - programmers, that is. webOS has a huge head start in the design area, so it neatly circumvents open source's achilles heel.
That's what I'm hoping, because the tablet space seems begging for competition. It's clearly the future of much of main-stream computer use. I don't even care if it gets all that much adoption. As long as the market isn't locked in by one vendor focused on native apps, it makes a strong argument for focusing on the HTML/JS open web experience a top priority.
On the flip side of the native app argument, he fact that native WebOS apps are HTML/JS also makes it a much more attractive target than it would be if it required learning another language. Even a modest percentage of that market would make that effort worthwhile for a web-based product.
An OS with a great interface is one hell of a head start for open source on tablets.