To be fair, "hacked" really isn't the right word. That word implies some kind of username/password cracking, which in turn implies some kind of secure system, and quite frankly, the All Star voting page set up by MLB is anything but secure. With a basic knowledge of HTML, a bit of Javascript, and a few minutes to play around, I was able to exploit MLB's All-Star voting system quite easily.
The key to exploiting the system was realizing that -- are you ready for this? -- there is zero verification surrounding the most important piece of information supplied in the voting process: your email address.