This question notes that a person briefly connected an Android device to an Apple computer via USB, and one comment indicates that it would "quite possibly" be a risk:
In theory, yes, in practice, probably not. If it had been a Firewire device, quite possibly. – Matthew
Why is Firewire a bigger threat than USB? How can connecting a Firewire device to a machine be a security risk?
I am a PC user and don't know anything about Firewire.