Ryan brings up the Wikipedia quality issue that's been buzzing around lately, and runs smack into the real problem - after noting, via Dave Winer, that everyone has an equal voice on the Wiki, we get to this as the solution:
Identify people who have expertise or knowledge on certain subjects
That's harder than you might think - and it all depends on the subject. I find that Wikipedia is pretty good on historical subjects (at least older ones), and that's because any controversy that may have existed on the subject has passed. For instance - look up Julius Caesar - the history reveals that there have been a number of reversions lately, but the general information looks pretty good - the damage on that page is the garden variety "I'm excited by curse words" sort of damage.
Now have a look at something more recent, and more contentious - the 2000 US Presidential election. Go browse the blogosphere if you think that there's anything resembling consensus on how that went down. I can't see there being a fully objective view of something as controversial as that election for a long time - it wasn't until deep into the 20th century that the 1876 election was viewed with any objectivity, for instance.
So back to the expertise question - how does a "real" encyclopedia deal with this problem any better than Wikipedia does? Take any controversial topic for which varying interpretations exist (i.e., nearly any historical event that happened within the last 100 years) - where do you find experts who have "unassailable knowledge" of some event? The bottom line is, you don't. Let's take a subject I've read a fair bit about recently - WWI. It's long enough ago now that some level of objectivity is creeping in - but it's still colored by subsequent events (WWII, the Cold War) - enough to generate controversy. What's definitive?
And that's just five books I've read on the subject - five books with very different discussions of how (and why) the war was fought. Let's take the encyclopedia up now - how does the entry on WWI address the war? How does it explain the hows and whys? I'll tell you how - it uses the (then current) academic consensus. Is that "correct" in any abstract sense? Who knows? It might be - or it might not be. The reality is, even WWI is still too controversial for there to be a reliable "consensus" view. Which means that the entry - whether it's in printed copy or bits - is just going to be some compromise view.
Exactly how does that differ for Wikipedia and any other work? It doesn't. The reality is, having "anyone" be able to edit doesn't mean that "everyone" will. Most people don't care deeply about any particular subject - the ones with an interest (and, of course, the vandals) will be the ones who show up. With the printed encyclopedia, anyone who's views fall outside the current academic consensus will just get cut out immediately. With Wikipedia, they have a chance to get their take peer reviewed and commented on.
Which leads me to the opposite view from Winer, and Ryan, and most other people - I'll take the Wikipedia approach over the standard. It's far more likely to allow a larger set of views fight it out.