Finally!
Miiverse will be integrated into the 3DS by the end of this year, Satoru Iwata has announced in the recent Nintendo financial results briefing. While you can now access a limited version of Miiverse through the web, Nintendo plans to flesh it out specifically for the 3DS.
"This will facilitate user communication about Nintendo 3DS games and help users enjoy them more deeply", says Iwata. "At the same time, the more people that take advantage of Miiverse, the more deeply and correctly they will understand the charm of the other platform, Wii U."