Regarding other details surrounding SIM unlocking, there is no real new information to present. All handsets sold from April will be eligible for unlocking even immediately after purchase. There will be a service charge for unlocking, but Docomo is still working out exactly the price. They are currently considering something in the ball park of "several thousand yen."
How will SIM unlocking effect handset price?
As usual, we have dueling opinions from Son and Yamada. Softbank's Son says:
さまざまな不具合が出てくる。解除した端末は単価が4万円ほど高くなる。While Docomo's Yamada says:
[SIM unlocking] will result in various problems. The price of unlocked handsets will increase
toby around 40,000 yen.
SIMロックを解除したからといって端末の料金が高くなることはない.。It media points out that there is more to consider here, namely the cost of the phone versus the total cost for the phone and service.
SIM unlocking is unrelated to handset price.
While I was at sea, Docomo announced a new "monthly support" plan（月々サポート）aimed at buyers of new smartphones. The deal appears to be that if you pay in monthly installments over 24 months, the exact cost you pay every month for the phone is discounted from your service fees. As soon as you've finished paying off the phone after 2 years, your monthly service fees will increase to normal, but since you are no longer paying for the phone, there will be no effective change in the price of you bill.
Docomo has not said if unlocking a phone would result in the cancellation of the discount. And because of this convoluted way of administering the discount - not discounting the phone cost, but the cost of service - even if Docomo were to cancel the discount, technically the price of the phone won't increase.
In the image below, on top are the service fees, on bottom, the cost of the handset in installments.