On the console version, you can jump in and out of the map as you're creating it, to see how it's shaping up and check things out. Was wondering if it's possible to do this with the Sandbox Editor.

Yes it is possible.
Hit ctrl-G

Follow on from Fabs

There will be some things that wont work in the editor when doing Ctrl+G such as the vehicles wont respawn and Assault Checkpoints wont work. You have to test these elements in the actual game itself.

Gotcha, thanks.

basically only the multiplayer stuff won't work. in the editor, its considered singleplayer.