I believe they are about the same, at lieast in your example of "think of something" / "think about something". "I think of you often" is exactly the same as saying, "I think about you often".\ "I'm thining of seeing a movie tonight" is exactly the same as "I'm thinking about seeing a movie tonight".

Thinking of something is more like CREATING a thought, a hypothetical maybe... like "Think of a situation involving..." Thinking about something is more considering it, thinking about it, imagining, reflecting: "I'm thinking about going somewhere." or "I've been thinking about you." BOTH can be opinions "What do you think of that?" "What do you think about that?" -- that's not a really strict difference but the other examples are noticable.

Think of = opinion Think about = process or detailed opinion