This is a jolly good question that has been continuously debated ever since Darwin proposed his theory of evolution in 1859 - and it's still a hot potato. For a start, it should be remembered that life in a substantial part of the undeveloped world is still a continuous struggle, so natural selection is as active there as ever. However, in the developed world things are more complex. Leading the case against the continuation of human evolution, is Steve Jones of University College London. He argues that natural selection is no longer effective as modern healthcare means that most people survive to a reproductive age and pass on their genes.
People now die of cancer and heart disease, which act later in life.
Natural selection is impotent against such diseases, as people who die this way have already reproduced, passing on their fatal inheritance. He points out that new diseases can still beat modern medicine and evolution selects for disease resistance, like AIDS for example. Environmental changes may also add new selection pressures. Christopher Wills believes that while there are still a few obvious cases of natural selection, most selection is subtler. For example the loss of fertility due to pollution means the child is never born and so it is not obvious that selection has occurred, but it has.