Nintendo announced today at the Nintendo DS conference in Japan a web browser for the Nintendo DS.

The Nintendo DS browser is based on Opera. It uses both screens and the touch panel input. The big screen shows a section of the page and the small one the whole web page. Text input is done via the stylus and handwriting recognition.
All this would make the Web browser superior to the browser available on the Sony PSP. The DS browser is no firmware upgrade though and costs money.
Nintendo will start selling the DS browser in June for 3,800 yen (~$32) in Japan.
More details on the Nintendo DS Conference slides (Japanese). See also the Opera press-release, which contains a bit more details about the DS Browser.