No code changes were made between 0.9.0 and 0.9.1. Only the dependencies were changed.
Upgrade OpenSSL to 1.0.1g. This release fixes the following vulnerabilities which can affect the Bitcoin Core software:
and
It is especially important to upgrade if you currently have version 0.9.0 installed and are using the graphical interface OR you are using bitcoind from any pre-0.9.1 version, and have enabled SSL for RPC and have configured allowip to allow rpc connections from potentially hostile hosts.