There are effectively three potential sources of vulnerability in the Bitcoin network: 1. A flaw is discovered in the Bitcoin client. 2. A flaw is discovered

There are effectively three potential sources of vulnerability in the Bitcoin network: 1. A flaw is discovered in the Bitcoin client. 2. A flaw is discovered in the public key cryptography algorithms the network uses. 3. An attacker has control of more than 50% of the processing power of the whole network. The first two vulnerabilities are common to any system dealing with finances. The third vulnerability is particular to Bitcoin, and would allow an attacker to "double-spend". This is because Bitcoin relies on a CPU-majority to determine which transaction came first, rather than a central authority such as a bank. Another point to note is that normal bank transactions can be reversed if fraud is detected, but bitcoins are like physical cash in that there's no automatic mechanism to void a transaction. There are ways of securely storing bitcoins, but the organisations and high-profile individuals who have been hacked have not had security even close to the level you'd want for storing hundreds of thousands of dollars.