Because Nintendo UK chooses not to dictate the retail price of any of its consoles, Nintendo is simply cutting the trade price for the DSi in the UK (meaning retailers will purchase the system for less). It's up to the individual retailer to pass the savings on to the consumer (or not) -- though most retailers will want to cut prices fairly consistently with the market standard to remain competitive. MCV estimates that the retail price of the DSi would drop from the current £150 ($220) standard to around the £120-£130 ($175-190) mark.
Though we'd expect a similar announcement from Nintendo of America, the company told Joystiq that "We have nothing to announce on this topic at this time." Maybe it's saving it for E3?