The math department is quite good. It's unusually large for a college of Carleton's size, with about a dozen professors and about 15-30 majors in each year. Math majors work closely with one of the profs and a small group to do research (though there are some other options) for their senior comps projects. You can see this year's projects at
Carleton College: Mathematics: Math Comps 2009-10. The Budapest Semesters in Mathematics program is a popular study abroad for majors.
Math majors have some very good post-Carleton options. Math grad programs students have enrolled in the past few years include U of MN, Wisconsin, UVa, Northwestern, UCSD, UPenn, Brandeis, and Indiana. For statistics or biostatistics, I know of recent math alums at Harvard, Stanford, and U of MN. A few very tip-top pure math programs (more or less Princeton, Harvard, MIT) might be out of reach for Carleton students because those expect significant graduate-level coursework when applying, but beyond that, Carleton students place very well. Actuarial work, consulting, and teaching (Teach for America in particular) are the most popular non-grad school paths. Most math majors take some economics or computer science courses at some point (or even double major with those fields), which helps with employability.