The short answer is yes, the long answer is no.
The short answer deals with the better utilisation of computer resources within the data centre, the potential for allocating virtual resources to more energy efficient services and for reducing the massive sprawl of physical resources (and all the energy hence consumed in manufacturing, cooling and distribution.). Overall, cloud computing offers the potential for more energy efficient virtual resources.
The long answer concerns componentisation. The shift of common and well defined I.T. activities from products to standard components provided as online services should lead to a dramatic explosion in innovation.
Standardisation always creates this potential.
If you consider writing an application today, the reason why it's a relatively quick process is because we have relatively standardised and stable components such as frameworks, databases, operating system, cpu, memory etc. Imagine how long it would take to write a new application if you first had to start by designing the CPU. This is componentisation in action, the rate of evolution of system is directly related to the organisation of its subsystems.
Cloud computing is all about providing standard components as services (it's pure volume operations). The problem of course is that we will end up consuming more of these standard components because it's so easy to do so (i.e. in old speak, there is less yak shaving) and it becomes easier to build new and more exciting services on these (standing on the shoulders of giants).
We might end up providing more efficient virtual resources but we will end up consuming vastly more of them.
In the short term, cloud computing will appear to be more green, in the long term it will turn out not to be. However, that's to be expected, our entire history of industrial progress continues to be about the constant creation of ever more complex and ordered systems and the use of stable subsystems simply accelerates this process, whether they be bricks, pipes, resistors, capacitors, databases or whatever.
Whichever way you cut it, our constantly accelerating process of creating greater "perceived" order and the constant reduction of entropy (within these systems and the future higher ordered systems that will be created) ultimately requires one external factor - a greater energy input.
Cloud computing will be no different and our focus will have to be on the source of that energy.