Rachel from Katipo [1], bringers of Koha [2], had interesting things to say on the Koha list regarding their decision to GPL their codebase, and lessons learned for others confronting that choice. Among others: "For Koha going open source and GPL has been the making of the project." Her message [3] responds to this question [4].
Links:
[1] http://www.katipo.co.nz/
[2] http://www.koha.org/
[3] http://lists.katipo.co.nz/public/koha/2002/000432.html
[4] http://lists.katipo.co.nz/public/koha/2002/000431.html