The only big difference between Atom D510 and D410 is that the first one has two cores while latter has only one.
Despite enormous similarity, Intel has decided to push Atom D510 for a Q4 2009 launch while the D410 single-core is still scheduled for Q1 2010 launch. Intel has realized that the market wants much more dual-cores than single ones in the desktop/nettop market, and this might be a good reason why they are planning to follow up with the single-core at a later date.
Engineering samples of the new single-core Atom D410 should be ready in week 36, or in late August, while production should start in very late October, but the launch plan is still unchanged and remains Q1 2010. Intel wants enough time to print enough of these CPUs.