I use IntelliJ 7 on a Mac at home and the shortcut for the code completions is listed wrong. The menu shortcut is shown as "-tab" (actually a little character with a right pointing arrow pointing at a vertical line). The shortcut is of course "-space".
Of course this assumes the cheat sheet I have for mac keyboard symbols is correct which shows the right pointing arrow pointing at a vertical line as tab.
On a side note if you use a Mac and use Quicksilver you will have change Quicksilver's default hot key or you won't be able to use code completion in IntelliJ.