From the NCBI Announce List:
Starting this month, we will begin displaying formulas in citation titles, abstracts and keywords in PubMed.
Previously, formulas were replaced with “[Formula: see text]”. With this enhancement, you will now see formulas in the PubMed summary and abstract displays when these data are available in new citations.
We will also be including the MathML 3.0 element tags in PubMed XML. To support the addition of MathML tagging in our XML, we have created a DTD, which you can download now. Existing content will be valid against the new DTD. You can also download sample XML files with MathML 3.0 tags.