Can a thief unlock or otherwise access the data on my stolen Android device? If my Android device is stolen, will the person who ends up with it be able to get my data by plugging it into a PC and using some trick? Is there anything I can do to prevent this or make it harder?
If you used full-disk encryption, then your data will be accessible until the device unmounted the encrypted disk (e.g. by rebooting) and discarded the decryption key. If you store your sensitive data in encrypted container, then your data will be accessible until the application closes the container and discarded the decryption key. 

If you don't use any encryption, then your data is naked in the wild. Many thiefs are simply interested to sell the phone to make a quick buck and so will simply reformat the phone; but if the thief has a specific intent to steal your data (e.g. corporate espionage, identity thief, etc), then you can assume that they can read all your unencrypted data.

If your phone uses SD card (instead of internal memory), then getting to your data in the SD card is trivial. Getting data in internal memory took a little bit more effort, but is still relatively easy. A turned off device with full-disk-encryption is -- for all practical purpose -- a useless brick for your adversary.