In addition to that, Iran is a signatory of the Nuclear Non-Proliferation Treaty, which allows them to develop nuclear technology for peaceful purposes. Israel is not.
The revelation of this Iranian nuclear facility is earlier than it needed to be under the terms of the NPT, which state that nuclear facilities must be revealed 6 months prior to becoming operational. Estimates (both by Iran and outside agencies) range from 1 year to 1.5 years before the facility is operational.
Iran has apparently agreed to inspectors:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=102x4078445Inspectors would make it impossible to use the site for weapons development (unless they blatantly kicked out the inspectors and converted the facility - the difference in requirements between weapons grade and power plant grade fissile material is astronomically high). Even if they still harboured a secret program, it would be severely hampered by inspectors. North Korea is a good case in point. Just before they kicked out the inspectors, the intelligence agencies predictions were that they were a decade or two away from a nuclear bomb. Then Bush labeled them part of the Axis of Evil, NK ratcheted up their rhetoric in response, and the situation escalated. Eventually, NK kicked out the inspectors, unsealed the nuclear material, and began actively working on a bomb. Within a few years, they had one. The lack of inspectors allowed the North Korean regime to greatly accelerate their nuclear weapons program. And keep in mind that there isn't any hard evidence publicly available that Iran has a nuclear weapons program (there's the usual accusations but there were similar accusations about Iraq's WMD programs and we all know how those turned out).
In short, with Iran a member of the NPT and seemingly willing to work through more diplomatic avenues, there are more and better options to handle the issue than just going with the knee-jerk "they're a threat and must be stopped at all costs" attitude.