If there has been a lot of time between Doomsday and now (as time seems to run differently in Pete's world) and Rose has moved on (married Mickey?) and they have a grown-up friendship (in S2 Rose was getting annoyingly Buffy-ish at her worst) then fine. I'm really not sure what the point of Rose's return is though? The only thing that does cross my mine is that they are going to regenerate Tennant at the end of S4 and the three specials thing in 2009 is all an elaborate smokescreen...