Actually, there has been a change in plans. Hong Won-pyo, the president of Samsung's Media Solutions Center, says that bada is not a fit for today's modern smartphones and as a result Tizen will no longer merge with bada, but will merely "absorb" its best features. Once Tizen flavored phones are released, the two will come together and bada will be nada more. Samsung plans on releasing a number of Tizen powered models this year.
After the absorbtion is complete, Tizen models will be able to run apps designed for bada but those with a bada phone will not be able to update to Tizen. And the big question is Samsung's intrention with Tizen. There is some speculation that Sammy is trying to distance itself from Android, perhaps in fear that a successful launch of the Motorola X could hurt Samsung's current status as Android's favorite Son.
source: YonhapoNewsAgency (translated), TheVerge via Engadget