Spanish publication Nintenderos says that the Official Nintendo Magazine in Spain has Super Smash Bros for the Nintendo 3DS down as being released this September. You may remember that US retailer Target had the game penciled in with an August 29th release for the United States. So we could be looking at September for Europe and August 29th for the United States. Hopefully we will receive a release date for the game next month at E3.
Update: The magazine says it made a mistake and they actually meant to write summer, rather than September.