I understand that, at the moment, if you encrypt your phone, it is the same password to decrypt (when you turn on your phone) and when you unlock.
I have read that Google is working on this to allow you to use a separate unlock code, e.g. the unlock pattern.
However, bear in mind that your phone is only as secure as your easiest code. If a thief obtained your phone while it was on, usually it is quite easy to crack a pattern lock (because of marks on the screen).
But if the phone is turned off, the thief would need a full decryption.