Version 1.50 of os-prober contains a license clarification, the
license of the package is GPL-2+, see for example the following commits:

http://anonscm.debian.org/gitweb/?p=d-i/os-prober.git;a=commitdiff;h=249d0814ae9c6e308baaaad4ec4e0a403a592ffd
http://anonscm.debian.org/gitweb/?p=d-i/os-prober.git;a=commit;h=25024daeccd86cec17353455b99af677b3b83b16
http://anonscm.debian.org/gitweb/?p=d-i/os-prober.git;a=commit;h=cc0ce8e5ec252b10127ca3ebb71c12cd31d4ebc4

The copyright file reads as of 2011-12-19:

The majority of code in os-prober is Copyright 2004-2011 by Joshua
Kwan, Joey Hess, Christian Perrier, Colin Watson and Otavio Salvador.
This is licensed under the terms of the GNU GPL, either version 2 or,
at your option, any later version.

Some portions of os-prober by other contributors has an unclear license
of "GNU GPL", with the version not specified.

On Debian systems, a copy of the GNU General Public License is available in
/usr/share/common-licenses/GPL.