Zero-cost collars?

[ericp]

The sample ?s for derivative markets, #1 has three threads here which I've gone over. I don't understand completely why a put option can be at the money but a call option can't be. If for a zero-cost collar that is also zero-width, both the put and the call strike prices are the same - at the forward price. Is it impossible for the forward price to = spot price? Because if that is possible then the call can be at the money, just like the put is.
If the spot price is, say 100 and the strike price for the put is 100 (at the money) then the only way the premium for the call is going to equal the premium of the put (for zero cost) is if the call has the same strike price (ie. also at the money).
The only way I can make sense of this is if it is impossible for the forward price to = the spot price. Since I have never traded derivatives I don't know what is and is not possible relative to the forward price.
Can someone help me understand why the put but not the call?
Thanks!