I understand that Roman Catholics do not think that everything the Pope says or does is infallible. However, has a sitting Pope ever revoked or contradicted an infallible statement made by a previous Pope?
You have asked specifically on an infallible statement, so the answer is NO. That is in the matters of Faith and morals which applies to the universal church, they have not contradicted each other.
But on other issues like when they make disciplinary and administrative decisions they have. For example, Pope Clement XIV suppressed the Jesuits in 1773, but Pope Pius VII favored them again in 1814.
Just to make it clear, it is important to keep in mind what Catholics mean by papal infallibility. It is a protection of the Holy Spirit which will not allow the pope to teach (as defined dogma) error in matters of faith and morals. Infallibility does not mean the pope receives private revelations. Nor does it include the private opinions of the pope, nor his teachings before he became pope--it is not retroactive. It does not mean he will say the right things at the right times, or lead a holy life. Papal infallibility does not extend to physics, algebra, or the outcome of sporting events.
Also it is to be noted that in the Catholic Church Church doctrines, disciplines, customs, and devotions each have different parts to play. In these Doctrines do not have to be formally defined by Church councils or popes to be part of the dogmatic teaching of the Church. But only when doctrines of the Church are challenged they are formally defined as dogmas by Popes.
To add a supplemental answer to Jayarathina's excellent answer:
When a pope speaks in an infallible way, this is called speaking ex cathedra. This is not a ritualistic linguistic formula or a "label" like nihil obstat, it is simply a way to recognize those times when the pope gives a teaching that has been correct, is correct, and will remain correct forever. The key point is that the teaching is not "new", but expresses a known and unchanging truth on a matter of faith (what should be believed) and morals (what should and should not be done). Ecumenical councils are another organ of infallibility.
Now, to be clear:
Note that ex cathedra is basically a clear statement on a particular topic, made by a particular person, and directed at a particular group of people. That's all. There have now been 266 popes over the course of 1981 years (32-2013). It does not take much to speak ex cathedra, and though it is not an everyday occurrence, many clear ex cathedra statements have been made. Given the enormous range of years, of different social attitudes, and of popes with different personalities and views, I personally think it is remarkable that Catholics do not have a huge pile of contradictory statements of this form. We should be very thankful.