Currently, the RRP of a DSi is £149.99, with some online retailers, such as Game, and Play.com, selling them for around about £120.
The announcement follows a drop in all DS prices in Japan, when Nintendo released three new colours for the DSi XL - Blue, Green and Yellow. The new DSi XL colours are due to be released in shops in Japan on the 19th of June, but as of yet, there is no announcement regarding a UK release.
The price drop presumably comes as Nintendo prepares to reveal their follow-up to the DS, the 3DS, at the Electronic Entertainment Expo later this month. The new console will apparently feature cutting-edge, jumping-out-of-the-screen 3D visuals, without the need for those incredibly fetching 3D glasses.