The zotero server code has been released quite a while ago. I understand it is not trivial (nor particularly well documented) to install on one's own server but has been done (search the forums and developer documentation). This is the only way to sync ALL of your data to your own server.
However, if the concern is with attachment data (as opposed to the library items' metadata) you can sync to your own WebDAV server, either by setting up a "normal" WebDAV server or using my php program, phpZoteroWebDAV, which is written in php and provides both a sync server for attachments and a viewer for both library and attachments. Note, though, that in the case of using a WebDAV server for attachment sync the bibliographical metadata of your items is still synced to zotero.org
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However, if the concern is with attachment data (as opposed to the library items' metadata) you can sync to your own WebDAV server, either by setting up a "normal" WebDAV server or using my php program, phpZoteroWebDAV, which is written in php and provides both a sync server for attachments and a viewer for both library and attachments. Note, though, that in the case of using a WebDAV server for attachment sync the bibliographical metadata of your items is still synced to zotero.org