Vulnerability always exists. In fact Open Government will open flood gates of intruders who will transgress for the heck of it just to amuse their perverseness. Systems that exists today when compared to what existed several years back, then it can be observed that they have evolved from being monolith to heterogenous systems. This could happen because of the development of the mechanisms for secured internal system transactions.
Open Government has begun several years back, when Government brought in legislation for information assurance and dissemination. Moving forward, as Vivek Kundra puts it, need for governance in the public square is most desired now. This means more interactive dialogue with the citizens to foster better mechanisms for the citizens to engage with the government. However, this does not mean business transactions are being conducted in a vulnerable open system. Just that the results of the transactions that are secured are made transparently visible allowing for citizens to better understand the government activities for making informed decisions.
IT Dashboard and many other contract details (http://www.fbo.gov) were made available on the internet for a long time now. Entire world could read and understand what the US Government was doing and also inference could be formed about the future intensions.
Recent cyberattack from (avoided "by") China on Google and like-wise attack on the Georgian twitter bloggers are simple examples of vulnerabilities. Also, one among the biggest fraudulent 2009 Gonzalez credit card case is another transaction related event. These cannot deter world from making progress. While the system strives for transparency, the notion of "privacy" will also be redefined. As more system opens up, that much more system will need to prepare for deterrence to prevent nefarious attacks. This is the anti-thesis. Security is a legitimate concern, but parents did not stop sending their vulnerable children to school. When that could happen mere discussion of the governance in the open is of less security concern.
The questions to be rephrased is - Does Government Fear to conduct in the Open, (Is it citing reasons not to) ?