Stephen Gornick - good point. I guess this would only work if person A paid coins to person B. Person B held on to them for a long time. Person A then paid the same coins to person C.
Enochian - if subsequent transactions by any person protect my previous transactions, then my whole point was moot. I think that's what you're saying. I wasn't sure from reading
http://www.bitcoin.org/bitcoin.pdf. Is there a more detailed document?