It is generically known that the cell membrane is composed of a lipid bilayer with embebed proteins, but agreement on its organization has not been achieved yet. Lipid-rafts have been suggested to be regions in the membrane with distinct compositons that would act as platforms for proteins involved in intercellular signalling pathways. Dispite a big number of publications in this subject, doubt still exists regarding the existence and nature of these lipid-rafts, mostly because of the limitations of the methods used for isolating this domains, in which small changes in procedure from work to work lead to different material isolated and consequently diffent material characterized. Until techniques that alow the marking and visualization of these assumed domains in living cells, the question will remain open.
Science Magazine is running a special about the Mysteries of the Cell, in which the lipid rafts between other intriguing unsolved questions about the cell are also discussed. More info here: