The issue of Mass Amateurization came up in a client discussion. Thought I'd pass along the links and thoughts I'd collected so far. Sorry for the endnotes, but I wrote it for ASCII email.
Here's Clay Shirky's blog posting that (seems to have) first articulated the concept. He had this to say about the amateurization of travel planning:
Mass amateurization is the web's normal pattern. Travelocity doesn't make everyone a travel agent. It undermines the value of being travel agent at all, by fixing the inefficiencies travel agents are paid to overcome one booking at a time. Weblogs fix the inefficiencies traditional publishers are paid to overcome one book at a time, and in a world where publishing is that efficient, it is no longer an activity worth paying for.
Tom Coates then expanded the concept to "(Weblogs and) The Mass Amateurisation of (Nearly) Everything..." For example, he states:
But it's not just publishing or journalism that are going through a process of mass amateurisation at the moment. In fact over the last fifteen years or so pretty much all media creation has started to be deprofessionalised. We only have to look around us to see that this is the case - as individually created media content that originated on the internet has started to infect mass media. Hard-rocking poorly-animated kittens that once roamed e-mail newsletters (http://www.b3ta.com) are now showing up in adverts and credit-sequences, pop-songs written on home computers are reaching the top of the charts, weblog commentators in Iraq are getting columns in the national and international newspapers, music is being hybridised and spliced in the home for competitions on national radio stations. The whole of the mainstream media has started to look towards an undercurrent of individual amateur creation because of the creativity that's bubbling up from this previously unknown swathe of humanity. Mass-amateurisation is EVERYWHERE.
And Ted Leung correctly applies the same concept to open source:
Tom Coates notes that just about everything is becoming amateurised. I think that there are parallels with open source. ... I wish that we could find better terminology. After all, the software running 64% of the web servers on the Internet was written by "amateurs".
Perhaps a better word is semiprofessional: everyone becomes semiprofessional across several professions.
A different perspective on the same phenomenon goes by various names: "hacks", "home hacking", and "consumer hacking." As the editor of the O'Reilly "Hacks" series explains:
What happens if you expose the rich mountain of Google data to hackers via a programmatic interface (the Google Web API)? How do you turn a store-front like Amazon's into a syndicated e-commerce engine? (Take a gander at the Amazon Associates program for the answer.)
Think about it. eBay has turned legions into amateur merchants. 40% of eBay's listing come through its Web services APIs, of which about 20% are initiated by 3rd parties. Amazon's APIs tell a similar story. The same thing is happening with advertising. Google's AdSense has turned everyone's personal web sites into ad revenue generating machines. All you have to do is insert a sliver of simple HTML boilerplate into your page design (and free up some page real estate), and whenever that page is displayed, Google will automagically post ads that are directly relevant to the content on the page!. For a case study in the airline biz, check out SeatGuru.com. It a favorite of mine because it lets you see the seating configuration for all aircraft models across the major airlines! No more first or last rows! Google even uses the site as a case study.
This ability for consumers to customize their software-driven devices and appliances and even cars is what accounts for the popularity of ring tones ($3 billion business), skinning, and car hacking.
The phenomenon has even been written up in the Wall Street Journal. A related phenomenon, user exchanges, where users exchange their extensions or add ons, has been around for a long time. And here's this month's Communications of the ACM with a special issue on "End User Development" aka "Do it yourself" (DIY) development. 
Note, in some ways this mass amateurization trend is nothing new. Note that all managers became amateur "secretaries" when word processing made it possible for everyone to do his or her own typing. Did it eliminate secretaries? No. They went on to do more complex things. It did put a big dent in typing pools though. <grin>
Update (2008-09-16): I originally misspelled Clay's name as "Clay Sharky"!