Someone should make a list of them (that goes beyond 50 projects). Without some sort of substantiation, there's no real reason to believe this claim.
Given that the list at Unlicense.org doesn't even include all of my own projects, it's really not comprehensive at all. So, here's a tip: just set a couple of Google Alerts (in addition to the obvious "Unlicense", searching also for the first line of text from the Unlicense is particularly effective). Much easier.
Note also that there isn't much point listing hundreds of projects directly on the front page of Unlicense.org. If we wanted to actually be comprehensive, we should do something like a tagged and searchable database of projects. This has been discussed previously, but isn't a priority.
For the other direction, that is, incorporating some copyrighted code into an otherwise copyright-free code base, just be careful to keep the copyrighted files isolated and clearly marked as encumbered, so that nobody accidentally copies anything from them into the unencumbered portions of the code base. See http://groups.google.com/group/unlicense/browse_thread/threa... for more information.
My sincere thanks to those who worked hard on the Unlicense! :)
