2.5.X breaks PS/2 mouse

One of my 486 laptops has a special port on the side, for a dedicated trackball.  I always assumed that it was electrically identical to the standard PS/2 mouse port, which is also present.

With 2.2.x, (I haven't tried 2.4.x on this machine), both the dedicated trackball and a standard external PS/2 mouse work fine, using /dev/psaux.

With 2.5.40, however, the behavior is as follows:

Booting:

With no pointing devices connected, just the port is detected:
serio: i8042 AUX port at 0x60, 0x64 irq 12

With the dedicated trackball connected, it also gets detected:
input: PS/2 Generic mouse on isa0060/serio1
serio: i8042 AUX port at 0x60, 0x64 irq 12

Same with a PS/2 mouse, it also gets detected:
input: PS/2 Generic mouse on isa0060/serio1
serio: i8042 AUX port at 0x60, 0x64 irq 12

Once booted:

Hot plugging either the dedicated trackball or a PS/2 mouse generates a message on connection:

Just to clarify, I am not trying to use them both at the same time.

X also works with the external PS/2 mouse, but not the dedicated trackball.

Any suggestions?

John.