Change is unlikely to cease once GPLv3 is released. If new threats to users' freedom develop, we will have to develop GPL version 4. It is important to make sure that programs will have no trouble upgrading to GPLv4 when the time comes.
One way to do this is to release a program under “GPL version 3 or any later version”.
Even though BusyBox code, as a whole, can only be used under GPL version 2, some individual files may have more permissive licenses: "GPL version 2 or later" - meaning that you can also reuse the code from this source file for a project which is distributed under GPLv3, and "Public domain" - the code in these files have no licensing restrictions whatsoever.