Razib takes exception to the Shi'a-Catholic/Sunni-Protestant analogy, and makes some very good points about why the mapping is not 1:1. However, as a Shi'a myself, though, I still find the analogy useful when discussing religion with Christians, because there are some string parallels that I think Razib overlooked:
Sunnis and Protestants focus more emphasis on the individual as the route to God. These can be loosely termed "democratic" though of course that allows for all sorts of grassroots-driven diversity. However that has its downside, as the rise of Christian and Islamic fundies alike who take a literalist view of the holy book, manufacture religious sources to support their interpretations (think hadith and historical revisioninsm ala "America was founded on Christian values") .
Shia/Catholics are more hierarchical driven and see te path to God as lying through an intercessor who has divine authority. This imposes a discipline of the range of interpretations possible of the holy sources. Shi'a dont accept any hadith that doesnt have an isnad (record of narration) that traces to Ali AS, for example. The role of Imam and Pope ar not equal but they certainly are analogous.
Note that Iranian and Iraqi Shi'a (and other Ithna Ashari "Twelver" groups) are very similar to Sunnis in the way they organize their religious authority structures. My own sect, the Ismaili Bohras (separate from the Ismaili Aga Khanis), is more traditional and thus follows the Catholic analogy (without equivalence) more closely.
As always, I urge anyone interested in learning more about Shi'a practices and beliefs as well as my own community's response to the challenges of integrating modernism and tradition, to read Jonah Blank's excellent ethnography, Mullahs on the Mainframe.
UPDATE: TheBit has been very active in the discussion here and now has devoted a full post on the topic up at Muslims Under Progress blog. I have onlyt two minor quibbles, but I think they do have a non-negligible import:
1. Shi'ism is considered the fifth madhab (validated by Al Azhar soe decades back) on equal footing with the "other" Sunni madhabs.
2. Using Iranian and Iraqi Shi'a as the sole example is not truly representative. The Ismaili branch, while numerically smaller, has actually had a much larger role historically. The analogy between Ismaili Shi'a and Catholics is actually very good, whereas between Ithna Ashari Shi'a such as Iranians and Iraqis it is not (since the latter have adopted a quasi-Sunni methodology to jurisprudence, as TheBit and Razib have noted).
Razib also has an interesting commentary on the act that I find the analogy useful, even though I agree with him that it is inexact. I should expand upon where I find analogy useful - specifically, in my (Bohra) emphasis upon the Dai ul Mutlaq as the sole interpreter of the faith.
The point I try to convey with the analogy is that religion is not democratic, and Catholics understand this immediately, whereas Sunnis (like Protestants) still argue that centralization of religious authority is a bad thing. Catholics may not hold the Pope's dictums in the same mandatory regard as we Bohras do for our Dai, but they still comprehend the reason better after invoking the analogy than before.
If the discussion ever proceeds beyond that point, then I do indeed abandon the analogy and begin to discuss the detaills.