I have a new Unibody MacBook Pro with a multi-touch trackpad.
As it came with 10.5.5 I was still able to "disable the trackpad when mouse is connected". As I did this, and subsequently moved to 10.5.6 I couldn't uncheck the disable function and my trackpad was unrecognized when ever a mouse or tablet was connected.
Now I rolled back to 10.5.5 and disabled the disabling (trackpad and mouse both work when mouse is connected) and when I updated to 10.5.6 again (to have my wireless N in a/b/g/n enabled) the trackpad still works with a mouse connected.
But the option to "disable the trackpad when mouse is connected" is gone in 10.5.6.
So whatever your last setting was in 10.5.5 is taken to 10.5.6 and can't be reversed.
I send Apple feedback about this, and the more people do, the more they could be inclined to restore that feature in 10.5.7, as it seems to a only affect multi-touch enabled Macs, since my my white MacBook with 10.5.6 still gives me the option to dis-/enable.