Between Nov 14, 2007 and early December 2007, MEDLINE citations added to PubMed will be "in-process" only, i.e. bibliographic information will be provided without Medical Subject Heading (MeSH) terms. Searches using MeSH terms will not retrieve newer literature until end-of-the-year processing is completed.
This occurs annually while PubMed switches over to the next year's vocabulary. You can begin using 2008 MeSH terms in December.