Sam Altman (Reluctantly) Says Y Combinator Has An Accelerator Monopoly
techcrunch.com
techcrunch.com
With startup school coming to London accelerators elsewhere will definitely be starting to get worried, but for now YC is simply 'best of breed', not a monopoly.
Google has a near monopoly on search. YC definitely does not (yet) have a monopoly on the concept of acceleration or even the start-up scene in general.
The reason why they might achieve a monopoly at some point is because of the network effects in the alumni.
Aaaaand Hacker news :).
No seriously, HN does a great job of reminding people to apply at YC, this probably helps them get lots of applications. Other advertising endeavors probably help (tour buses going directly to Harvard, MIT, etc. where YC alumni talk to folks).