http://ds.ign.com/articles/103/1039028p1.html
"A new Nintendo DSi with a larger screen will be available by year's end in Japan, reports website Nikkei.
The report states the screen will now be close to the size of Sony's PSP, nearly double that of the current DSi screen area. This will not only allow for higher display of graphics and text, but possibly new applications as well, such as reading electronic books and watching video.
The new DSi will be priced at 18,900 yen ($205 USD), the same as the current model at retailers. The report states Nintendo is considering releasing the new DSi overseas.
We've contacted Nintendo of America for a response and possible release. Keep checking back."
It's been nearly a year since the DSi was released in Japan, so timing wise, this isn't too bad. I'm not sure whether we'll see it outside of Japan, but if so, I'm sure there will be a long wait.
EDIT:
Quick update. Here's an article that discusses the idea of 4" screens on a DS and what it might look like: http://kotaku.com/5391387/what-would...ur-inch-screen
Also, rumors of it being called a DSi XL or LL (depending on country sold in). No confirmation on that...