Well, the binaries in fact might be runnable without re-compilation. It really doesn't matter at all from compatibility point of view whether a re-compilation is needed or not as long as both platforms offer the same API, which in case of Maemo and MeeGo is the Qt API.

But for distributing the apps need to be repacked (MeeGo .rpm <-> Maemo 5 .deb) But again this does not matter for an average user installing the applications, the steps necessary are already taken by somebody else when the app is installable in the repo (the application developer, Nokia, community, ...).