I'm not sure if there was a reason, but the page (<!--nextpage-->) Quicktag button was "removed" in 1.5.1. To get it back, open wp-admin/quicktags.js in a text editor and locate this section (starting at line 135):
/*
edButtons[edButtons.length] =
new edButton('ed_next'
,'page'
,'<!--nextpage-->'
,''
,'p'
,-1
);
*/
Just remove the /* and */ lines to uncomment the block of code and return the button to your editing windows.