To make the pen-tester's life easier, a new type of security software has been developped : Vulnerability Scanning tools. The most famous ones are Nessus, Saint, and Satan. Those software automate the research for vulnerabilities on systems by scanning their ports and matching the results with their knowledge database. That makes such programs able to detect potential threats to the system/network.
But lately, CORE SECURITY TECHNOLOGIES have developped a brand new type of tool : an Intrusion Automation System[14, Core Impact]. This software scans for vulnerabilites AND tries to hack into the machines. If it succeeds, it tries to infect other machines (a bit like a virus), so it does more or less the same thing as a pen-tester (in fact, it uses the same pen-testing algorithm described above).
Still, even if such software is much more powerful and intrusiv, it can't replace a pen-tester IMHO for the same reasons as I mentionned for vulnerabilities scanners, so all those tools are much more helping the pen-tester than being a replacement solution.