Dojo sets the 'autocomplete' attributes to 'off' by default. When manually adding these to the textbox, it doesn't work.
Change the onclientload event on your xpage and add this code :
dojo.attr(dojo.byId('#{id:username}'), "autocomplete", "on");
dojo.attr(dojo.byId('#{id:password}'), "autocomplete", "on");
But somehow this doesn't work in IE and Chrome...