If apple controlled a majority share of the consumer pc market, this scenario could possibly be the case. But they don't, so there's room for the mac to expand.
There's still room, therefore for an ipad halo effect. Parents buy kids an ipad, marvel at the design and ios, etc. Their laptop or desktop later dies, becomes obsolescent or they get tired of scrubbing malware off of it. Mom and Dad may then remember the little macbook air they saw at the apple store or best buy when they bought the ipad...
That said, I have a 3g ipad and a 2 year old aluminum macbook with maxed out ram and a 7200 rpm hd. The alMB handles my relatively simple consumer needs quite well, and I have no plans to upgrade or replace it unless it dies out of warranty. Period.
OTOH, I may CONSIDER a new ipad if the v2 is 'all that and a bag of chips' or has a feature that can enhance my work. If it just has cameras and can pull up a web page 15% faster, I'll probably wait for the v3.
I don't think they'll introduce a new version that is dramatically better than the original, or people will (rightly) suspect that they intentionally hobbled the first one to force people onto the upgrade path. It will have improved specs and one nifty feature to encourage people still on the fence, or the 'newest fastest and shiniest' crowd, to buy one.