Juan Cole on Iraq's Grand Ayatollah Sistani's recent condemnation of Israel's bombing of Lebanon and demand for an immediate cease-fire. Moqtada al-Sadr has already indicated his willingness to send Mahdi Army fighters to join with Hezbollah, and as Cole notes, Sistani needs to proect his flank from the steadily growing power of Sadr.
The Sadr family is originally from Lebanon, and Moqtada's cousin, Sayyid Musa al-Sadr, is one of the pivotal Lebanese Shi'a leader of the twentieth century. It was Musa al-Sadr who issued a fatwa (religious legal decision) in 1974 declaring Syria's Alawites to be Muslims, a decision which is still considered controversial among mainstream Sunnis and Shi'as.
It's important to place the current conflict within the context of the rising tide of Shi'a political activism in the Middle East, which the invasion of Iraq threw into high gear. The fact that the most effective Arab military organization in the modern era is Shi'a cannot be overlooked or underestimated, nor can the fact that Hezbollah is also solidly Khomeinist in its ideology. Sadr is a professed Khomeinist, Sistani is not. Given this connection, along with the resonance of the Sadr family name in Lebanon, it's quite clear that Hezbollah's success benefits Sadr at Sistani's expense, and thus makes it somewhat more likely that the United States will have spent trillions of dollars and suffered thousands of casualties (and inflicted hundreds of thousands) in order to transform Iraq into a very hardline conservative jurisprudent-dominated state, instead of merely a moderately hardline conservative jurisprudent-dominated state.
Further reading: A very good New Yorker essay on Hezbollah from a few years ago.