This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.