Yes, that is the way it is supposed to work. I just called my local store (and they said they are saving me one!) and the rep verified that I could sign up for a prepaid plan, use it for the month and then not do anything for 3-4 months later and there would not be any activation fee when I decided to use data again.
Now I believe it is a month from the day you purchase - so use it or lose it in that 30 days. So actually, now that I think about it, I'll probably be buying just about every month for travel! Damn.....