Well, I'm assuming that the PC would be turned off and the side taken off before being put in the fridge (I'm thinking a fridge, freezer would get much colder). That way, all the particles that might freeze would do so without harm to the PC. Also, I've never seen ice particles in my fridge. I'm assuming it doesn't get below 0°C in there, so I don't think any freezing would be an issue.
Although you're right in saying that when you take the PC out, condensation would occur. But again, I'm assuming that the PC would be turned off, therefore causing no damage to components.
OR, we could just cover the whole motherboard in Dielectric grease, or AS5 or something, like those DICE and LN2 users do.