I've just got notification of the arrival of a pre-ordered Kindle ebook on Amazon UK.
I pre-ordered
A Mighty Fortress by David Weber on 8th April at a price of £5.59 — just 6p cheaper than the paperback that was due out as well.
My notification today told me that I purchased the ebook for £4.68. The current price shown at Amazon UK is £5.38. The paperback is still £5.66 (Amazon discounted price).
Good to see that Amazon's pre-ordering price promise actually works - you get the product at the cheapest price it's been offered between your pre-order time and the release date.