She's trying to suppress a fart. Cheri Oteri who used to be in many comedy bits with Will Ferrel on Saturday Night Live always had that facial expression.
Now that someone pointed it out, Obama does look like Curious George. However, the t-shirt isn't exactly a step in the right direction, towards racial peace. Then again, neither was Rev. Wright's speech...LOL
Just an observation and kind words of advice...women over the age of 30 should NEVER hold there face like that because it shows off the dreaded double chin.