order by date desc but NULLs last

I have a simple query that sorts by descending date but the NULL dates 
show up first. Is there a way I can sort so they come last without 
sorting ascending?

SELECT submittime,score,gradedtime FROM student_gradedmaterial WHERE 
gradedmaterialid=3 and studentid=102 order by gradedtime desc;
     submittime      | score |         gradedtime
---------------------+-------+---------------------------- 2003-01-30 22:56:38 |       | 2003-01-31 03:42:29 |    99 | 2003-02-06 14:21:43.043587

but what I want is all the graded items first in gradedtime desc and 
NULL afterwards. I do need to keep the NULL score rows. (So I get the 
latest submitted grade for the assignment but also any ungraded 
submission information.)

Thanks for any advice.><><><><><><><><><
AgentM
agentm@cmu.edu