Again, via Phil Dokas.

Hold Control with Cmd+Shift+3 or 4 and your screenshot will be copied to your clipboard without generating an image file on your Desktop. Paste right into Photoshop.

Phil Dokas followed up with:

⌘⇧4, click + drag to make a selection without releasing, now hold down the spacebar and move the cursor.

…

If you instead hold down spacebar + shift, you lock one axis of movement.