Hi - I work on the Photosynth team, and this question comes up often, so let me summarize our thinking. Cloud synthing sounds great, and will definitely be the way to go if and when we tackle the problem of synthing directly from mobile devices.
For desktop computers though, we've discovered that in almost cases people's upload bandwidth is such that uploading takes longer than the actual synthing. Since we do both simultaneously right now, there would be usally be no time saving for our users if the synthing were done in the cloud.
Secondly, as Nathanael says, the compute required for a big synth (~300 images+) is really substantial, and as you've probably noticed, it scales non-linearly. (i.e. synthing 400 images takes more than twice as long as synthing 200 images). Doing this with in the cloud would get very expensive, very quickly, as we scale up.
But, there *are* clever things we can do in the cloud. We're definitely working on this. Stay tuned!