I have a 60D and have never had this problem before although I have never set the exposure for 30 minutes I am so disappointed its a great shot
Even without seeing the picture, I'm 99.9% sure that's noise from the long exposure. The camera indeed allows you to do it (bulb mode would even allow you to expose for an indefinitely long time with just the battery running out cutting off the exposure), but the sensor itself is not really meant for that. As you begin the exposure, the camera's sensor is fed a certain voltage, which results in a certain amount of heat being produced by the sensor. Under "normal" conditions (exposure times in the order of fractions of a second to a few seconds) this heat can be controlled, but when dealing with orders of magnitude longer exposure, the same heat builds up to the point that it causes a false readout (i.e. interpreting information that is not there, i.e. noise).