If I recall- McCain announced Palin between the DNC and RNC...
I'm willing to bet that Romney will not wait until between the two conventions to announce his running mate.
(The RNC is before the DNC this year.)
He probably won't have a short list ready before mid-June at the earliest. He might announce as early as late June if he wants to get his running mate out on the stump as soon as possible, but I expect he'll either announce or leak the short list, and then after seeing how they perform for a month or so, he'll announce in late July or early August, so that his running mate can be well-prepared before the convention.
