From within a custom form control, there appears to be no way to reference the current document.

CMS.CMSHelper.CMSContext.CurrentDocument returns null

This bug can be reproduced within version 4.0 of KenticoCMS as follows:
- create a custom form control
- create a dummy field within any document type
- set the field type of the dummy field to the new form control
- from within any event of the form control attempt to reference CMS.CMSHelper.CMSContext.CurrentDocument

We are currently using a workaround:
CMS.TreeEngine.TreeNode currentNode = treeProvider.SelectSingleNode(CMS.GlobalHelper.QueryHelper.GetInteger("nodeid", 0));
but are already aware of cases where this will not work.