Not specifically outside the US, but the US (and apparently Canada) have the concept of copyright assignment, many other countries don't, so it gets interesting what value and consequences a copyright assignment by a programmer from there has, esp. in local courts and when the contributor isn't on your side.
Don't forget that "moral rights" may not be assigned at all, in at least some countries, so you'll need a per-country agreement that the contributor won't enforce them ... in countries where it is possible to agree not to enforce them, which is not all countries.
