It's not exactly illegal. While there are stipulations surrounding the practice, sign stealing is, at it's core, one of those gray areas in baseball that leads to a vigorous discussion of what's over the line.
Earlier this month, Rockies pitcher Jamie Moyer accused Chipper Jones of the Atlanta Braves of stealing signs, leading to an on-the-field verbal altercation between the two players. As recently as this week, the Blue Jays were accused for the second time in recent memory of relaying their opposition's pitching signs to batters during a game.
Currently there is no written rule that forbids sign stealing — as long as you don't use any foreign equipment, like binoculars or a telescope.
Some would argue that sign stealing is wrong altogether, while others will tell you it's all a strategic part of the game. With the Red Sox visiting the Rogers Centre on Friday night to play the Jays, the question may be more important than ever.