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.
Performance-wise and functionality-wise they're identical: they compile to the same code.
I like having the imports at the beginning, so you can see at one glance what parts of the Java API (or 3rd party libraries) a class needs. It also gets repetitious: note how in the second example you have to type the package name twice. Having the imports up front also helps the IDE help you type class names (assuming you're using an IDE).
The most common case where I don't import a class up front is if a class uses two other classes of the same name, like java.awt.List and java.util.List. That confuses the compiler, and you need to take a bit of care to get things straight. Usually I end up importing the one used more often, and fully qualify the other one each time I use it.