After, as Earthbound/Mother fansite bar none Starmen.net says, ten years, Mother 3 has been dated in Japan by series creator Shigesato Itoi. The date: April 20, 2006.
Mother 3 has been in the works for quite some time, most famously appearing in 1999 at Spaceworld for the Nintendo 64. No plans to release any games from the series (known as Earthbound stateside) have yet been announced outside Japan.