How to toggle commenting and uncommeting code in IDEA?

Yes, highlight the block and type Ctrl + Shift + /. To uncomment, just repeat.

Jetbrains provides a printable reference card with the default set of Windows and Mac key bindings.

For the benefit of others who might stumble across this question & answers... In addition to the windows key binding provided in other answers, there is also a keystroke for toggling a comment bock in IntelliJ IDEA on the Mac.

Line Comment

⌘+/

Block Comment

⌘+SHIFT+/

with Shift this makes for block style comments /**/, without it for line style comments Ctrl + / -> \\ (tested with IDEA 10U)