Since there's a big huge picture of the returning aliens on the home page of BBC's Doctor Who site declaring they're returning in A Good Man Goes to War I saw no need to spoiler code anything. But just in case someone is trying to maintain total secrecy, the third returning alien has been announced by the BBC.
The Sontarans
Whooo, that should be quite interesting but didn't I read something prior to series six that no bad guys from series 5 would show up in series 6 save for the silence?
The only thing I read is that there were no plans to use the Daleks in S6 or to re-introudce any classic era aliens that hadn't yet been in the nu show. Everything so far has been consistent with that claim.
I suspect that cost is the real rationale behind that decision. If they've been in the nu series, the costumes etc have already been made and can be re-used thus getting more use out them and not having to fork out for new costumes.