OASIS Universal Business Language standard. Not a far reaching standard, but probably more useful for small, resource-starved development projects.

From the standards document:

... The OASIS Universal Business Language (UBL) is intended to help solve these problems by defining a generic XML interchange format for business documents that can be extended to meet the requirements of particular industries. Specifically, UBL 1.0 provides the following:

Updated Sept 17 2008: new links

One of the great contributions to the world of business rules in the last decade is the OMG Model Driven Architecture for Business Rules. The OMG link will take you to a page listing several formats for the article. I suggest using the PDF format for Business Semantics of Business Rules, it's about 240K. They also have MS Word and RTF formats.