A lot can happen in three years.
Really a candidate for 2012 will have to announce within a year (by ~ March 2011) to avoid the Fred Thompson fate, which I see as the Romney/McCain-nightmare scenario. It goes : (2011) Palin doesnt announce, Romney is leading in polls, Palin is pushed in by supporters too late October 2011, ...Romney gets it, conservative voters stay home, Obama wins
But to your point about things changing, it makes me wonder why there appears to be a civil war on this current hypothetical scenario(Palin vs Romney) given we know things will change in a year or so.