I am not sure what Israel hope to accomplish against Hamas in Gaza with the current conflict. Israel surely do not want a repeat of the last Lebannon war. Then, it might have tactically weakened Hezbolla but the resultant military stalemate became a political loss of Israel. I am not just sure that Israel can take out Hamas even if Israel take over Gaza. Certainly this does not mean it is worth the effort to inflict massive damage on Hamas.
What is more potentially interesting is that Israel may be, through the attack against Hamas, may actually provoking Iran, Hamas' major backer, into some sort of response. A military response by Iran would certainly justify an Israeli attack against Iranian nuclear program.
All this is happening just before Bush leave office (a more supportive PotUS for Israel) and potentially laying the ground works for an Obama administration support.