Sharing an AppStore download across computers
Technically, the press says it will be possible and supported for businesses and schools, but you may need to pay for volume licenses. It's just not yet clear if this will be economical or feasible for individual consumers.

In general, you will have to download each purchase once for each mac. Updates will come up as small delta packages to reduce the load on you as well as Apple. Lion itself seems to be getting extra handling as a special case.