Well, even with the fuel rods melting, it still isn't likely to spread radiation globally. So, yeah, this sucks... but it's definitely NOT Chernobyl.*
Is this really another Chernobyl?
In a word, no. The situation in Japan is different from Chernobyl, which had a completely different nuclear power plant design. Chernobyl had an inherently unstable graphite-moderated reactor, not an inherently stable water-cooled reactor. In addition, all western (modern) reactors must have a containment building; Chernobyl did not have one. Therefore, even if the reactor core in Japan were to melt, the primary containment, a steel liner surrounding the reactor core, should limit the release of radiation. However, we do not know if the primary containment was weakened by the earthquake. Nevertheless, the longer the time between shut-down and collapse the better the outlook because the reactor generates less and less heat with time.
The reason Chernobyl exploded was because of the materials that were used to cool the core. Daiichi does not have this same problem, and in addition, there are other measures in place to contain any potential fuel leakage. So, sorry, no poisonous gas cloud will be hovering over the horizon any time soon.
*Just in case no one has said this often enough.