Tom Christiansen posted the "Leap Year FAQ" on slashdot.
The original seems to be http://world.std.com/~dpbsmith/leapyearfaq.txt. This FAQ is reasonably impressive and, as far as I'm concerned, settled the question just as nicely as some of the other FAQs from the United States Naval Observatory and the other folks who keep track of dates and times.
Knowing some Latin, I thought it would be fun to give a translation of the passage from Gregory. If you find a mistake in my translation, please let me know. Here we go:
Deinde, ne in posterum a XII kalendas aprilis aequinoctium recedat, statuimus bissextum quarto quoque anno (uti mos est) continuari debere, praeterquam in centesimis annis; qui, quamvis bissextiles antea semper fuerint, qualem etiam esse volumus annum MDC, post eum tamen qui deinceps consequentur centesimi non omnes bissextiles sint, sed in quadringentis quibusque annis primi quique tres centesimi sine bissexto transigantur, quartus vero quisque centesimus bissextilis sit, ita ut annus MDCC, MDCCC, MDCCCC bissextiles non sint.Then, in order to prevent the (vernal) equinox from receding away from the 12th day before the start of April in the future, we have commanded also that the bissextile in the fourth year (as it is the custom to observe) ought to be continued, excepting in the hundredth years; although in the past they have always been bissextiles, and thus also we wish the year 1600 to be, nonetheless those of the hundredth years which follow thereafter shall not all be bissextiles, but in all intervals of four hundred years the first three hundredth years shall go by without a bissextile, yet each fourth hundredth year shall be a bissextile, so that the years 1700, 1800, and 1900 shall not be bissextiles.
Anno vero MM, more consueto dies bissextus intercaletur, februario dies XXIX continente, idemque ordo intermittendi intercalandique bissextum diem in quadringentis quibusque annis perpetuo conservetur.Moreover, in the year 2000, according to the usual observance, a bissextile day is intercalated, with February having 29 days, and the same rule for interposing and intercalating a bissextile day in the four-hundredth years is observed in every year forever.
Intercalation is the insertion of something, traditionally as of a leap day into a calendar.