Nintendo's official comment states, "In the UK Nintendo will be reducing the trade price of Nintendo DSi to retailers from June 18, 2010. The ultimate price to consumers is, of course, determined by the retailers." It is not yet known how much Nintendo is cutting the system's price for retailers.
The DSi's current recommended retail price is £149.99, but the system can already be found for about 30 pounds cheaper online.
Thanks to stepthreeprophet for the tip.