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.
something == null will be turned into an assignment if you forget one =. Now usually you would get a compiler error stating that the expression is not boolean, but if something is declared to be Boolean, auto unboxing will turn it into a boolean and it will compile just fine. It won't run though, since the unboxing will throw a NullPointerException.
If you use null == something, it will always be a compiler error if you forget one =. You can't assign anything to any literal, including null.
I prefer something == null though - if you test properly these kinds of errors will be quite clear.