It is call free flight mode.
Press F on the keyboard to go to that mode, Press G on the keyboard to go back to top down mode.
Holding the middle mouse button and sliding it will go into freeflight mode also. The indicator is in the upper right of the edit view window.
Hold the shifht key and the right mouse and the ASWD keys to move around more smooth.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit