The IRS contends otherwise; from http://yorba.org/docs/IRS-determination-letter-final.pdf, page 8:

Software is not a public work. The development and distribution of software is not a public work even if published under open source or creative commons compatible licenses because software is not a facility ordinarily charitable to include "erecting or maintaining public buildings, monuments, or works." This language slightly broadens the original formulation from four centuries ago.

It goes on to quote the Charitable Uses Act of 1601.