The biggest diff that stands out to me is the word "common" and the word "new". Unifying for a change.

I notice that Obama speaks on many more topics than Bush. Bush's cloud is less dense than Obama's, meaning he used repetition to try and convey a message instead of speaking to each message individually and relating it to the whole. These clouds also show me that what Bush was trying to accomplish wasn't achieved, because people still need the same message. 
 
Disclosure: Yes, I voted for Obama and despise Bush. ;)

Great work, Mike! I had seen this use case when researching Wordle, and am glad someone took the time to do it. I look forward to the commencement of real work by the new administration as well.

They are very similar, but that's what makes over-analyzing the slightest differences so much fun. 
 
 
 
The words that made me go "hmmmmm": 
 
 
 
generation  
 
common 
 
president 
 
crisis 
 
God

This is one thing that does bug me about the internet and attention to specifics in this way. 
 
 
 
I am not an Obama junkie. I see where the words used are very similar. However, I would argue that the context and overall spirit/meaning of both speeches were very different. As Americans it is important that we listen to all of what our elected leaders have to say instead of relying on soundbites or specific words or word instances. 
 
 
 
Of course I am impressed that you took the time to do this. :) 
 
 
 
The common thread here is one person elected to lead our nation, the difference is more subtle. Pay attention America!

I did a similar experiment on my blog, although I didn't think to compare the two speeches. An interesting experiment. The high level themes do seem similar, but - at the risk of cherry picking words and phrases - I do see some telling differences in the less frequently used words. 
 
With President Bush, I note a couple of variations on terror, attack, faith-based, evil, and so on. With President Obama, it's crisis, ambitions, challenges, greatness, cooperation, and courage.  
 
As you and others here suggest, the words they say may be similar, but the truth, I'd suggest, will be revealed in the execution of those words.