Currently, the SharePoint Permission Checker web service (permcheck.asmx) does not support item-level security. It only supports list-level security. Checking for user permission for each individual item through the existing permcheck.asmx implementation would be too expensive from a performance perspective (especially for large SharePoint lists). A solution for this may be to change the existing call to the List web service to a custom SharePoint web service (could be an enhancement to permcheck.asmx) to pull content for a specific user.