I cannot tell you in very concrete terms what we're doing (it has to do with data integration), but I can tell you the reason why we will probably NOT do it as a web app. It's not the UI capabilities, it's the data hub problem and economics.

Anything you do on the web has to funnel every piece of data through that web app because browsers cannot access anything other than their originating server (They might soon be able to do that via Flex/Silverlight type plugins). Even if they could, it just doesn't make sense for many data centric interactive applications to buy a lot of servers when a huge number of powerful PC clients does almost nothing.

I reckon, if I have some algorithmic data analysis stuff and I can distribute that to client machines to some degree, I can make my service cheaper and maybe even architecturally simpler in some cases, because there is a natural partitioning scheme. Ok, now I see this argument becomes quite obscure without going into further detail so I'll leave it at that.