The following excerpt from the Schema.xml file for custom lists defines the New command of the toolbar, displaying the option to create items only to users with permission to add list items.
<Toolbar Position="After" Type="Freeform">
<IfHasRights>
<RightsChoices>
<RightsGroup PermAddListItems="required" />
</RightsChoices>
<Then>
<HTML><![CDATA[ <table width=100% cellpadding=0 cellspacing=0 border=0 > <tr> <td colspan="2" class="ms-partline"><IMG SRC="/_layouts/images/blank.gif" width=1 height=1 alt=""></td> </tr> <tr> <td class="ms-addnew" style="padding-bottom: 3px"> <img src="/_layouts/images/rect.gif" alt=""> <a class="ms-addnew" ID="idAddNewItem" href="]]></HTML>
<URL Cmd="New" /><HTML><![CDATA[" ONCLICK="javascript:NewItem(']]></HTML>
<URL Cmd="New" /><HTML><![CDATA[', true);javascript:return false;" target="_self">]]></HTML><HTML><!-- _locID_text="onetid6" _locComment="{StringCategory=HTX}" -->$Resources:core,addnewitem;</HTML><HTML><![CDATA[</a> </td> </tr> <tr><td><IMG SRC="/_layouts/images/blank.gif" width=1 height=5 alt=""></td></tr> </table>]]></HTML>
</Then>
</IfHasRights>
</Toolbar>