I have just released version 1.0 of Statistics: an Objective-C Foundation framework inspired by Perl's Statistics::Descriptive. By that I mean it has a similar interface, though is not a straight port.
Straight out of the box this framework provides count; min; max; range; mindex; maxdex; biased and unbiased variance and standard deviation; mode; median; percentile; arithmetic, harmonic, and geometric mean; and frequency distribution, optionally cumulative. In addition you can return a new statistics object seeded with the same data but with outliers discarded. Perfect if you're interested in the trimmed mean.
The code is released under the BSD license.