1. Like Hamas (and unlike, say, al-Qaeda), Hizballah has a constituency for which it is responsible. Were Hizballah to attack Israel, it would not want to do so as the result of a hasty decision-making process. Although Hizballah indeed attacked Israel in 2006 at the same time Israel was active in Gaza, that was an operation that had been planned (and attempted, unsuccessfully) for some time. Were Hizballah to attack Israel now, the IDF -- with its ground forces still engaged in Gaza -- could still make life miserable for the Shia population of Lebanon. Keep in mind that in 1993 and 1996, Israel was able to displace well over 100,000 Lebanese from the south on each occassion solely through the application of air power and artillery. In 1993, over 6,000 homes were destroyed. In 1996, more homes than that were leveled -- along with four civilian power plants. So were Hizballah to attack Israel, that would probably be the result of a very deliberate and calculated decision. Because it would be understood that the Israeli response could be devastating.
2. It is possible that Hizballah would very much like to attack but has been counseled by either Iran or its Lebanese political allies not to do so. Again, why this would be the case remains a mystery to me -- and would to all but a handful of Iranian and Hizballah decision-makers.
3. Although the IDF is busy in Gaza, I am quite sure their defenses along the Blue Line are on full alert and expecting an attack of some sort. Could Hizballah launch a few rockets south? Of course. But I think they might have more difficulty pulling off a more spectacular operation like the one on 12 July 2006.
Of course, contra Matt, reason #2 is consistent with Hamas* and Hezbollah being closely tied to Iran and being at least in part extensions of Iran's regional policy towards Israel (admittedly, it does disprove some of the more zombie-like arguments wherein Iran single-mindedly pursues Israel's destruction and Hamas and Hezbollah have no independent agency). Fortunately, reason #1 strikes me as the most likely option. Reason #3 is undoubtedly true as far it is goes, but there is no question in my mind that Hezbollah -- if it set its mind to it and didn't care about the inevitable Israeli response -- could wreak some serious havoc right now even without something as ambitious as its 2006 operation.
* Do people view Hamas as an arm of Iran? I would have thought that the Sunni/Shi'ite split would make that rather difficult. I have no doubt that Hamas and Iran have contacts and connections, but I can't imagine Hamas going so far as to render itself an Iranian client organization.