I assume some form of multi-selection and bulk edit is planned, but I am mentioning it anyway.
To multi-select items, I think the regular Windows approach is usually quite good - ctrl-click and shift-click.
To bulk edit, I think RTM is probably the best app I have seen, if I remember correctly. In the task edit pane you can see clearly (visually, using graphical means such as dimming etc) for each attribute whether 1) all or 2) none or 3) some of the selected items have that attribute. This info serves as a useful warning. If you change the attribute it will be changed (set or cleared etc) for all selected items consistently regardless of each item’s initial value for that attribute.