CategoryInfo ci = CategoryInfoProvider.GetCategoryInfo("Development", "CorporateSite");
string where = CategoryInfoProvider.GetCategoryDocumentsWhereCondition(ci, true);
TreeProvider tree = new TreeProvider(CMSContext.CurrentUser);
var documents = DocumentHelper.GetDocuments("CorporateSite", "/%", TreeProvider.ALL_CULTURES, false, "CMS.Folder", where, null, -1, true, tree);
something like this should work for you ;)