  We present and discuss new importance sampling schemes for the approximate
computation of the sample probability of observed genetic types in the
infinitely many sites model from population genetics. More specifically, we
extend the 'classical framework', where genealogies are assumed to be governed
by Kingman's coalescent, to the more general class of Lambda-coalescents and
develop further Hobolth et. al.'s (2008) idea of deriving importance sampling
schemes based on 'compressed genetrees'. The resulting schemes extend earlier
work by Griffiths and Tavar\'e (1994), Stephens and Donnelly (2000), Birkner
and Blath (2008) and Hobolth et. al. (2008). We conclude with a performance
comparison of classical and new schemes for Beta- and Kingman coalescents.
