WiFi turns itself off when 3G is enabled

[porkchop9000]

I spend most of my time at home or at work where I can connect to wifi. I have a cheaper plan from Verizon but kept getting dinged with overage charges. I have noticed that sometimes when I am at home or at work the 3G would be active and when I checked my wireless settings, wifi was OFF. I started disabling 3G unless I was on the road. When 3G is disabled I never find wifi off, but when I forget to disable 3G, I find this happening again. I suspect this is why I kept going over my plan. Is this a known issue? It almost seems designed to cost me more money.