How can I free 'hidden' hard drive capacity?

I'm in the market for a new notebook hard drive. Replacing the old one is easy enough, but here's a thought: I've seen drives that offer 120GB and others that offer 160GB, yet both have one platter and two read/write heads. So, are they identical?

If they effectively are, can I buy the (cheaper) 120GB drive and access the 'missing' 40GB?

PS. I know these are unformatted capacities, but surely there has to be a way of accessing the 'hidden' capacity, no matter what it is.