This has come up before, though not really a big discussion. Personally, it depends how bad the cold is. I don't really get nasty symptoms, just drowsiness and a stuffed nose. If this is the case, I'll still go but will FAR from push normal weight/reps. Just take it easy, since ultimately you want to get well ASAP and a combination of strenuous exercise and being in an unhygienic environment like a gym is no way to go about it.
If I feel like a bag of shit, I won't go. I don't like not going, but hey it's necessary.