DOCBOOK-APPS: Underscore in titles and hyperref package

I try to transform a docbook document into pdf that has underscores in
chapter titles (e.g. Class_Name). I am using openjade 1.3, the DSSSL
stylesheets V1.70 and pdftex. The hyperref package, however, thinks that an
underscore starts a subscript:

Package hyperref Warning: Token not allowed in a PDFDocEncoded string,
(hyperref)                removing 'subscript' on input line 12000.

In the end the TOC is incomplete. I don't think the underscore a special
character as the HTML rendering (openjade/DSSSL) works fine. Is there a way
to 'escape' it in some way or to switch off hyperrefing in the generation
of pdf?

Thanks,
Ekkehard