I'm not sure what is going on there (I haven't been working on the
version of sagenb that comes with 4.8). However, the default place for
worksheets is:
~/.sage/sage_notebook.sagenb/
and I don't think that should have changed between 4.7.2 and 4.8.
You can specify the directory explicitly by using the directory parameter:
notebook(directory='~/.sage/sage_notebook.sagenb')
Thanks,
Jason