maciekrb, the web service is used by the synther every time a synth is made.
As far as creating or uploading synths using the webservice, all the compute is performed by the user's own computer while the photos are uploaded, so unless you can write your own bundle adjuster which outputs Photosynth's data formats, it's unlikely that you can use the web service to create your own synths.
I've always wondered whether Bundler could be made to export to Photosynth's format. There are plenty of third party tools to convert images to Seadragon's DZI format. Even if the DZIs, DZC, JSON file, pointcloud binaries, etc. were all generated and zipped up as they ought to be, you would also need some way to authenticate yourself to the service, which is to say some way to sign in with Windows Live ID so that the collection has some account to be filed under.