In the dataset for the Project you have read/write access to Task_WBS. Unfortunately this should be read-only and the documentation is being corrected. (See community comment already posted at http://msdn2.microsoft.com/en-us/library/websvcproject.projectdataset.taskrow.task_wbs.aspx#CommunityContent - and look out for the updated downloadable SDK next month). This can appear to work and you can see any changes written to the dataset and even persisted to the database and published to PWA views. However if the project is then opened from Project Professional 2007 any values perviously set via the PSI are ignored and reset (or just blanked out) according to the settings in Project Professional.
One potential workaround if you need to control the Task WBS values externally would be to use another custom field that you could control fully through the PSI.
This is also another reminder that you too can post community content to the SDK on MSDN.
Technorati Tags: Project Server 2007