For some reason, the <!--nextpage--> Quicktag button was "removed" in 1.5.1. To get it back, just do the following:

In Wordpress 1.5.x

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. That should return the button to your editing windows.

In Wordpress 2.0.x

Open wp-includes/js/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 two // lines after and before the block of code to uncomment it.

Now Open 'wp-includes/js/tinymce/plugins/wordpress/editor_plugin.js in a text editor and locate this section (starting at line 15):

var buttons = '<a href="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\')" target="_self" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\');return false;"><img id="{$editor_id}_wordpress_more" src="/{$pluginurl}/images/more.gif" title="'+titleMore+'" width="20" height="20" class="mceButtonNormal" onmouseover="tinyMCE.switchClass(this,\'mceButtonOver\');" onmouseout="tinyMCE.restoreClass(this);" onmousedown="tinyMCE.restoreAndSwitchClass(this,\'mceButtonDown\');" /></a>';
// Add this to the buttons var to put the Page button into the toolbar.
// '<a href="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpresspage\')" target="_self" onclick="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpresspage\');return false;"><img id="{$editor_id}_wordpress_page" src="/{$pluginurl}/images/page.gif" title="'+titlePage+'" width="20" height="20" class="mceButtonNormal" onmouseover="tinyMCE.switchClass(this,\'mceButtonOver\');" onmouseout="tinyMCE.restoreClass(this);" onmousedown="tinyMCE.restoreAndSwitchClass(this,\'mceButtonDown\');" /></a>';

On line 17 copy the text between the apostrophes and paste it at the end of line 15 exactly before the trailing '; as follows:

var buttons = '<a href="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\')" target="_self" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\');return false;"><img id="{$editor_id}_wordpress_more" src="/{$pluginurl}/images/more.gif" title="'+titleMore+'" width="20" height="20" class="mceButtonNormal" onmouseover="tinyMCE.switchClass(this,\'mceButtonOver\');" onmouseout="tinyMCE.restoreClass(this);" onmousedown="tinyMCE.restoreAndSwitchClass(this,\'mceButtonDown\');" /></a> <a href="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpresspage\')" target="_self" onclick="javascript:tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpresspage\');return false;"><img id="{$editor_id}_wordpress_page" src="/{$pluginurl}/images/page.gif" title="'+titlePage+'" width="20" height="20" class="mceButtonNormal" onmouseover="tinyMCE.switchClass(this,\'mceButtonOver\');" onmouseout="tinyMCE.restoreClass(this);" onmousedown="tinyMCE.restoreAndSwitchClass(this,\'mceButtonDown\');" /></a>';

This will show the the Quicktag <!--nextpage--> again. You can also enable the hotkey access by doing just one more thing. Open 'wp-includes/js/tinymce/plugins/wphelp/editor_plugin.js with a text editor. Note that it's not the same file as the one from the previous step, as although it has the same name it is on a different path. Locate the following lines (should be line 23 and 24):

+ '<input type="button" accesskey="t" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\');" />'
+ '<input type="button" accesskey="u" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'Undo\',false);" />'

Copy line 23, paste it as a new line just below line 23, and edit it as follows:

+ '<input type="button" accesskey="t" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpressmore\');" />'
+ '<input type="button" accesskey="p" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'mcewordpresspage\');" />'
+ '<input type="button" accesskey="u" onclick="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'Undo\',false);" />'

This will enable the Alt+P shortcut for the <!--nextpage--> tag.