I've always tried to be fair to Lamark when I teach - after all he did get much right. It now turns out that even his mechanism may work in some situations.
In the last few years the exploding field of epigenetics has finally provided the mechanism by which acquired characteristics can be passed on to offspring. (Epigenetics is the study of how gene expression is altered by mechanisms other than changes in the underlying DNA sequence.)
Here's one very readable introduction to the topic: Two new studies show that the effects of a mother's early environment can be passed on to the next generation.
or a more in depth paper on bacterial epigenetics:
Epigenetic Gene Regulation in the Bacterial World
UCSB has a number of researchers working on epigenetics and a number of courses you can take if this is of interest to you. Professor David Low of MCDB, one of the authors of the above paper, works in this area and has just agreed to become a CCS advisor. He has had very positive interactions with CCS students in his lab and hopefully this will continue.