It should be noted that Matt Cain and Barry Zito also wear Mizuno cleats so maybe they had an influence on him. We also checked the mizunousa.com baseball site, and Timmy wasn't listed as of one their players on the Mizuno roster. In addition, based on the April 18 photo, Timmy is wearing a Nike undershirt underneath his jersey. In seasons past, Timmy has typically "cut out" the Nike swoosh logo since he was under contract with Reebok. So our guess is that he hasn't signed a full endorsement deal with Mizuno - not yet anyways.
Timmy from April 18, 2011 - Mizuno cleats, Nike undershirt
Timmy with Mizuno
Timmy with Nike
Timmy with New Balance and Nike