I see two main targets: Grandmas (sorry to stereotype the tech savvy grandmas out there) or rather tech savvy sons of grandmas which want something that just works for their mothers and that automatically backs things up for them.
And enterprises who want to kick Microsoft prices in the nuts.
I see the Grandma use case, but it's so niche I can't see that being viable. As for enterprise, you'd have to have a pretty "progressive" company... and one that would be very sure that you'll never need a desktop app. Again, I would think very small numbers.
I realise Chrome OS is somewhat still a market experiment, but I think it would've gone a lot further at a better price point. For ~$200 I would put one in my living room for checking the web & controlling my home web apps. I see that as the perfect way to get people familiar with it (thinking of it more like a tablet, a limited utility), then once NaCl goes mainstream, you have a who market who's ready to start replacing their other PCs.
I can tell you that a huge international bank (as far as "progressive" as you can get) went Google Apps and ditched Office, so your thinking might be a bit off.
