Ocker, I agree that intervention is rarely ever successful.
In the recent spate of BOJ intervention by buying USD and selling JPY, it happened many times over a course of weeks. These sales were definately to weaken the YEN versus the Dollar (to assist balance of trades when imports are stronger than exports). This is obvious by the suddenness and magnitude of the sale (when the price reaches certail levels people expect bank intervention).
The difficult bit is, the banks never buy at exactly a given point or time, hence people get caught. Sometimes it's not even during the particular country's trading hours. In the recent case, most interventions increased the price of the $USD?JPY around 150 pips, but within a week they had retreated to previous levels. I guess it's more a case of stopping the trend rather than reversing it.
In the case of other reasons for currency sales/purchases (foreign aid or whatever), I don't believe it would ever be sold in one parcel. The reason is obvious...why force the price to a position that is detramental to you?
As for the info being up to 6 weeks old, most forex vendors will post news items within half an hour of the purchase. I admit there is no way of this being an accurate assessment, but as a ball park figure it's good enough, since in such sudden unpredicted moves very few others get the chance to jump on the train, both entry and exit stops tend to be jumped.
