To switch between Intel GMA 4500 MHD and ATI Mobility Radeon HD 3450 (M82XT), right click on battery icon.
When Photoshop CS4 is running, you cannot switch.
T400 C2D2.4-3 ATI HD GMA Gfx 4.6 3.9 Gfx.G 4.6 3.7
There is how it should look like in device manager
During the driver install, you may see errors like these. They are "normal".
You have to use Lenovo's switchable video driver (combo with both ATI and Intel drivers) for it to work. Currently, it is at version 8.563.2.1-090401a-079160C (2009.04.28), which put it 4 months behind ATI driver release.
9.5 beta 8.61
9.4 8.60 2009/04/08 OverDrive
9.3 8.591 2009/03/18 Unified driver for Windows 7/Vista WDDM 1.1
9.2 8.582 2009/02/20
9.1 8.573 2009/01/29 (full OpenGL 3.0 extensions)
8.12 8.561 2008/12/10 (Stream computing, NR for progressive video)
If you want those bug fixes in 9.x, supposedly you can do - Use Driver Heaven's mod tool to reconfigure ATI's installer so it will install. You will have to reinstall the Intel driver afterwards so ATI CCC can do switching for you.
There are other issues if you want to use the switchable in XP, Windows 7, Ubuntu etc. Usually involves going into BIOS and change
Display | Graphics Deivce : Integrated Graphics/Discreet Graphics/Swtchable Graphics
Display | OS Detection for Switchable Graphics : Disabled/Enabled