  We describe different methods for estimating the bispectrum of Cosmic
Microwave Background data. In particular we construct a minimum variance
estimator for the flat-sky limit and compare results with previously-studied
frequentist methods. Application to the MAXIMA dataset shows consistency with
primordial Gaussianity. Weak quadratic non-Gaussianity is characterised by a
tunable parameter $f_{NL}$, corresponding to non-Gaussianity at a level $\sim
10^{-5}f_{NL}$ (ratio of non-Gaussian to Gaussian terms), and we find limits of
$|f_{NL}|<950$ for the minimum-variance estimator and $|f_{NL}|<1650$ for the
usual frequentist estimator. These are the tightest limits on primordial
non-Gaussianity which include the full effects of the radiation transfer
function.
