I just installed bbswitch on the newly-released Thinkpad T440p.

Loading the bbswitch module and disabling the card works perfectly fine:

[  142.881587] bbswitch: version 0.7 
[  142.881593] bbswitch: Found integrated VGA device 0000:00:02.0: \_SB_.PCI0.VID_
[  142.881596] bbswitch: Found discrete VGA device 0000:02:00.0: \_SB_.PCI0.PEG_.VID_
[Package] (20130517/nsarguments-95)  
[  142.882097] bbswitch: detected an Optimus _DSM function
[  142.882106] pci 0000:02:00.0: enabling device (0004 -> 0007)
[  142.882127] bbswitch: Succesfully loaded. Discrete card 0000:02:00.0 is on
[  156.250136] bbswitch: disabling discrete graphics
[Package] (20130517/nsarguments-95)  
[  156.265409] thinkpad_acpi: EC reports that Thermal Table has changed
[  156.376985] pci 0000:02:00.0: power state changed by ACPI to D3cold

But enabling it again seems to mess up the PCI bus, crash the network adapters, and cause data corruption (files read from the disk contain random characters, filesystem errors, some files are missing, empty or filled with random data after a reboot).

[  160.406244] bbswitch: enabling discrete graphics
[  160.647323] pci 0000:02:00.0: power state changed by ACPI to D0 
[  160.647336] thinkpad_acpi: EC reports that Thermal Table has changed

There are no bbswitch errors, but shortly after entering the command, the syslog fills with various kernel messages related to internal devices no longer responding. The filesystem sometimes remounts as read-only, and the system becomes unusable and has to be reset.

At this point, the machine is unable to write files to the disk or a USB stick or communicate with the network, so I made some "screen shots" using my smartphone. I tried to redirect the syslog to another machine using the internal network as well as a USB WLAN adapter, but the data cuts off as soon as the graphics card is enabled.

Installing the other bumblebee components or the Nvidia/Nouveau drivers does not make any difference.