We thought so too. That's why we've started an inititative in the open source project to provide access to xProcess via Web Services. This will be a different server to the current xProcess web server (see screen shot) which uses Tapestry and has facilities for supporting participants on xProcess projects (booking hours to tasks, closing tasks with gateways, access artifacts and assigning tasks). There will be a considerable amount of common code however as the authentication, integration with Subversion, scheduling and multi-threading will all follow the same design. However the web services interface is intended to give full flexible access to the complete underlying data model of xProcess. The first services being defined allow you to access and update project data, but the mechanisms of data exchange and update are generic so almost anything is possible!
If you think this project might be of interest to your needs, or if you're interested in participating in it, please contact us. Your input will be most welcome. There are further details on the Wiki about how to use the source code once downloaded: http://sourceforge.net/apps/mediawiki/xprocess/index.php?title=WebServices