For studying invariant NKT cells, it is best to use alphaGalCer-loaded or PBS57-loaded CD1d tetramers for detection (available upon request from the NIH tetramer core facility, I believe), in conjugation with CD3, CD4, CD8, CD56 stains, and some B-cell (CD20)and monocyte (CD14) exclusion markers.
In addition to what Pierre said above, a novel monoclonal antibody (clone 6B11) is now widely used to phenotypically characterize human invariant NKT cells (Montoya et al., Immunology, 2007).