So, to imply that it might be possible to network boot -
Who is doing the implying? Two people have said, correctly to the best of my knowledge, that the RPi hardware is only capable of booting from the sd card.
OF COURSE it is possible that the code that is booted from the sd card could be some sort of minimal pre-loader that is capable of finding and transferring control to some other code, possibly an entire Operating System, located on some other storage device or even over a network connection.
BUT anyone capable of implementing that will not be deterred by the headline and simply understood statement: the RPi hardware is only capable of booting from the sd card.
Can't we agree to leave it at that and not muddy the waters?
How To Ask Questions The Smart Way: http://www.catb.org/~esr/faqs/smart-questions.html
How to Report Bugs Effectively: http://www.chiark.greenend.org.uk/~sgtatham/bugs.html