The problem is unfortunately not as simple as a naive first pass might assume.
There's a lot of nuance involved here - and the fact you have to do bi-directional sync, with a reasonable expectation of "real-time" makes it tough as well.
Also - the fact that other desktop clients (e.g. Dropbox, Box.net) exhibit similar performance characteristics (e.g. Dropbox advertises a 300,000 file limit - I believe this customer was going on 700,000 files) would also probably indicate it's not quite that simple.
There's a lot of nuance involved here - and the fact you have to do bi-directional sync, with a reasonable expectation of "real-time" makes it tough as well.
Also - the fact that other desktop clients (e.g. Dropbox, Box.net) exhibit similar performance characteristics (e.g. Dropbox advertises a 300,000 file limit - I believe this customer was going on 700,000 files) would also probably indicate it's not quite that simple.