Somebody tech-savvy enough with physical access to your place (a maid, a boyfriend/girlfriend, a roommate etc) could deliberately infect your computer while you're not around to get the PIN. Even a hardware key-logger would suffice. Once with the PIN, s/he only needs to steal the device.
This is exactly what I am worried about. I recognize the fact that some people prefer comfort when entering their PIN as opposed to an additional security precaution, but wouldn't it be beneficial to allow both methods of PIN input - on the device and/or the computer keyboard? The button layout of the Trezor makes it clumsy to punch in the PIN, but far from impossible. Many of us would sleep easier knowing our security could not have been compromised because of an infected computer. At the same time, those who prefer the comfort of their keyboards can simply not utilize this feature and enter the PIN as it was originally planned by the developers. It's really just a matter of modifying the software and I urge the developers to at least consider it.