Error creating thread

[exec]

I'm currently fiddling around with some stuff, and I was curious 
how well thousands of threads run simultaneously.
The problem now is, that I'm often running into the following 
exception:
core.thread.ThreadException@src\core\thread.d(817): Error 
creating thread

Here's my test program: 
http://pastie.org/private/qqkvbxxkkn7mw9gapqwkg

It only seems to happen with >1000 threads, and I'm seriously 
wondering what's the cause of this.