Double check locking is broken. Since initialized is a primitive, it may not require it to be volatile to work, however nothing prevents initialized being seen as true to the non-syncronized code before instance is initialized.
EDIT: To clarify the above answer, the original question asked about using a boolean to control the double check locking. Without the solutions in the link above, it will not work. You could double check lock actually setting a boolean, but you still have issues about instruction reordering when it comes to creating the class instance. The suggested solution does not work because instance may not be initialized after you see the initialized boolean as true in the non-syncronized block.
The proper solution to double-check locking is to either use volatile (on the instance field) and forget about the initialized boolean, and be sure to be using JDK 1.5 or greater, or initialize it in a final field, as elaborated in the linked article and Tom’s answer, or just don’t use it.
Certainly the whole concept seems like a huge premature optimization unless you know you are going to get a ton of thread contention on getting this Singleton, or you have profiled the application and have seen this to be a hot spot.