This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
I think the more important question is: why would you NOT want to allow null keys and values? Using null values for defaults (if there is no value for a non-null key, check the value for the null key) isn't uncommon. If you want to disallow them, use Collections.checkedMap - that one forbids null as well.