I'm not sure I follow your question...are you asking whether or not we have seen that if you have good SERP rankings in MSN, and then you begin to rank well for the same keywords in Google, your MSN rankings then begin to drop? Can you please clarify the question.

On a side note, I would say that I have found it relatively easy to consistently and quickly get cached and ranked in MSN, but I have also seen some pretty interesting results with google picking up my sites quickly, not just caching, but ranking them as well - I think there are a lot of factors that go into this...obviously.