I seem to recall the commentators at Money in the Bank talking about how the Money in the Bank winner could only challenge the Champion of the brand that that respective briefcase was for. Either way, having him cash it on Smackdown would be a huge mistake and I am sure that the WWE are not even considering this as an option.
Lets look at it this way, The Miz absolutely dominated Sheamus on Raw last night and it shows us that he could take Sheamus out at the drop of a hat. Given that he is the upcoming face of the company, why would the WWE want to move him to Smackdown where people wouldnt get to see him? Make no mistake, The Miz is going to be a WWE Champion at some point and by moving him to Smackdown, it defeats the purpose of having the different matches at Money in the Bank.
The Miz will not cash-in his Money in the Bank on Smackdown. I can say that with the utmost certainty. Why would he want to? Smackdown doesnt need him right now and he has proven that he can handle the competition at the top level of the WWE and that is on Raw. His attack on Sheamus taught me that The Miz is on Raw to stay.