Android - Can thief unlock stolen phone that was locked by Android Device Manager?

The answer is almost certainly yes.

Someone could probably get past it in one way or another. (They could probably just use odin on a samsung device for example. Though I have never done something like that myself.) If you have sensitive data on there then a remote wipe might be the best thing.

Did you report it stolen with your carrier? They could report the ESN / IMEI as stolen so at the very least the thief probably wouldn't be able to sell it.

Of course I do not know your exact situation. If you know for sure it was stolen and not lost could the police recover it from the thief?

Either way best of luck to you.

It would depend on the phone, but as firesoul453 said in his answer, almost certainly yes.

Locking the phone is intended to protect the data - In other words: the thief cannot get your data off of the phone. Due to the nature of the android system though, it's usually very easy to wipe out the data on the phone. Usually this can be done via recovery, or even by bootloader unlocking the phone, neither of which require the PIN to be entered.

Additionally, if you are on a version of Android prior to 4.1 and have USB Debugging enabled, they can completely disable the lockscreen PIN without having to wipe the data.

If it's still connected to Android Device Manager, though, they haven't wiped the data yet.