Historically, a fork has been seen as a sign of trouble in a project, and folks have gone to huge and painful means to prevent forks. Heck, not long ago the blogging rage was about which licenses helped prevent forks, BSD-style or GPL-style. Things are changing. It isn't really that new, though...
Aside from the big-obvious (Linux, which has encouraged forking, well, forever (in internet time)), even staid centralists like Apache talked about it as a good and natural thing. In that context it wasn't supposed to be a fork, though, it was supposed to be "a new version", and it stayed in the same source tree and was one big happy project.
At ApacheCon, in New Orleans, Dirk-Willem asked the key question, when discussing git, subversion, etc and the ASF -- "are we shaped by the tools, or do the tools shape us?" Many leapt to say that we shaped the tools, of course. It is, of course ;-), a blend. The tools shape the mindset which shapes us who shape the tools.
Forking is painful because merging changes between forks is painful. It is seen as a huge duplication of effort, a squandering of developer time, and a "I'm taking my ball and leaving, thbbft!" For some high profile things, such as X.org, this has been kind of true. Many times it can also e irreconilable goals (such as Dragonfly). Many times, however, it is just "I need something, but a little different, and the best path for me right now is to take something and incompatibly change it" (nginx, for example).
Git (and others, but whatever, I am talking about git, and hwile others support many-repos, only git really expects and encourages many repos) removes much the effort part from the forking equation. Github, in particular, as Chris makes clear, allows for very easy moving of code from fork to fork. Linus, a git I have never met, wrote software to support how Linux devlopment works (ie, with Linus as the SCM) automating what he did, and along the way opened the tooling door for everyone else.
Personally, I love the "please fork my stuff" mindset. I write open source stuff to solve my problem, and hey, if it solves yours too, bonus, have fun, don't sue me.. The bonuses add up, which is why I bother to do it in the open -- people do take it and run. I get patches, good feature requests, and whole ports of code I wrote and actively disowned a couple times a week. I always hate telling folks "I no longer maintain that, some guy named Chris, or maybe Anthony, took over, I think -- he hasn't made a release yet, though, so not sure what is up." If it is a bugfix someone sent me, I have done the spelunking to apply the fix and cut a release on code I expect to never use again (I take bugs personally), but aside from that it is "er, yeah, svn is here I think, let me know if you want commit."
There are also projects I use every day and are in a stable state (jdbi for example). It works great, I know other folks use it, but frankly, it is just a utility library. If you need changes, don't wait on me! Git, and something like github to make it even easier, means I can say "here is the code, fork it, go to town." A month later when I have spare time I can pull changes back, or if the forker really takes the ball and runs with it, I can just start using that code.
Where I think it doesn't work for people is when they have strong financial or emotional attachement to their code. If you make your living consulting on OpenWombat and uppity twenty year olds go and fork it all over, your job just got tougher. Worse, one of these forks may become more popular than OpenWombat and you won't now own it.
My personal opinion is to let the best code win, and the best code is a moving target. Chris's example, in the conversation with David, assumes the most active code line is the best code. Sometimes this is true, but more often (in my opinion) the more stable (not abandoned) code line is probably better. This is kind of Linux vs FreeBSD (and Linux clearly has won the popular vote) but... FreeBSD is solid as a brick in a way I doubt Linux will be for a long time yet, and stabilty trumps features for a lot of things. In a free-fork world, this difference of opinion is easily resolved, and the best features of the unstables can flow into the stables much more easily.
I am fascinated to see the shape of how this evolves. There are huge social and legal potholes ahead on the new fork, but in the end, as we have all proven, the best code does tend to win despite the efforts of trolls and zombies. Lowering the barriers to contributing and experimenting leads, more or less directly, to more development and experimentation, so I expect that the best code will tend to emerge from the network of forks which make sharing changes between them as easy as possible.