not replacing my mbp, replacing my ipad
The ipad market is exactly the market I think that is more endanger of changing their minds and going for the MBA instead.
The MBA seems more aimed at people who want a supplemental second computer for ultra portability than the MBPs which seem aimed some at that crowd but also at people like me who want their laptop to also be their main computer (and therefore willing to sacrifice some size for more capability).
For me my 13" MBP is the perfect balance between capability and portability (though to be honest I wouldn't want bigger) for what I want in a computer.
I'd definitely not replace my MBP with one, but if for some reason I was considering an ipad, I'd probably be more tempted to spend a little extra for the 11.6" and get something that had a real keyboard and a full OS for really not much more size.
Though, I am not sure how well the 13" MBA will fare. It doesn't have enough of a size difference between it and my MBP that I'd think would make up for the power difference (I'd have a hard time seeing going for it when for just a little more thickness you could get more outlets, a DVD drive, a bigger hard drive with capability to install a far bigger hard drive, more RAM, a faster processor, all for just 100 dollars less than the Air). Honestly, I bet the 13" does as well as the last MBA.
But I think the 11.6" will do well. But I don't think it was aimed to replace MBPs but more as a supplemental computer (as this post proves. Not one person so far is planning on replacing their MBP with one but a few are considering adding the Air as a more portable second computer).