After upgrading to GWT 2.0, my project suddenly stopped compiling. Almost 4 hours into the problem, but its finally solved!
I was using the Base64 class in Apache Commons Codec library for some base64 encoding. GWT includes the commons codec package but an older version of Base64 which didn’t have the encodeToString(byte) method in it that I was using. Had to reorder the jars to get it working again.