There are three games in the Mother trilogy -- Earthbound is technically Mother 2 -- so the compilation could include the first two titles, the latest two titles (Mother 3 was released in Japan for GBA earlier this year), or all three titles. So far, only Earthbound has been released in the US.
Despite EGM's general reliability when it comes to rumors, the continued popularity of Earthbound, and the Ness love shown in Smash Bros, we are not getting our hopes up just yet. Nintendo recognizes the popularity of the series in the US -- they would be blind not to notice -- but the likelier outcome is Earthbound being made available on the Wii's Virtual Console, and possibly downloadable to the DS.
[via Nintendo Wii Fanboy]