The more salient graph would be by penetration of the Mac platform, rather than Quarter-by-Quarter sales. Sales _levels_ are just now back to recovery, but your point about old Mac shops clinging to their OS 9 machines indicates that much of the sales volume in the last few years has gone to "switchers" -- which grows the overall base, and that base is the fundamental driver of sales projections looking forward. If I switched for the first time, say, 3 or 4 years ago I am probably now about ready to buy my second (or third) Mac, and when the Intel machines start coming out I suspect you'll see a lot of folks buying their second Mac. Those buying their second Mac seem like the real "canary in the coalmine" for the platform, because they represent switchers who are staying with the platform. Bottom line: it might not be as bleak as you suggest.
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