[Craig]

To make Return make a new row instead of saving:
- OmniFocus > Preferences... > General > Quick Entry: uncheck "Return should also save and exit"

To make a new row if you want to keep Return to mean save and exit:
- hit Cmd-Return instead of Return.

[edit: this last one appears to work no longer in current builds (though you can still Shift-Return to insert a row above, rather than below...)]

[edit again: Ctrl-Return is what I was looking for]