Do you not think Google's datacenters are more secure than your home? Or the Train? Or Starbucks? Also, do you trust whatever encryption and network security you have in place more than you trust Google's?
This is an interesting article on the matter:
The Park Paradigm - Urban myths in clouds.
Bottom line for me: it's in Google's interests not to lose data. It's in my interests not to lose my laptop. Difference is, I won't/can't devote 24 hours per day and billions of dollars to make sure that doesn't happen.