Circe is correct.
People often get confused about the differences between academic degrees (BSN, MSN, PHD, DNP, etc), certifications (CNS, NP, CCRN, CRNA, CEN, etc), and licensures.
Degrees are granted by colleges and universities.
Certifications are granted by accreditation agencies (ANCC, AACN, etc)
Licensures generally come from State Boards of Nursing.
To be eligible to sit for an Advanced Practice Nurse certification examination, you need to have completed a specialty program from a program that is accredited by NLN or CCNE. These specialty programs are done at the MSN, Post-MSN, and DNP levels. At present (September 2009), you can enter into Advanced Practice with a MSN degree.
The American Association of Colleges of Nuring (AACN) has called that entry into Advanced Practice require a doctoral degree by 2015. Some Universities have already made this transition and have eliminiated the MSN programs. Other Univerisities are maintaining their MSN programs for now. Other universities have no intention of opening a DNP program and hope the 2015 call from AACN is dropped (or goes the way of the 1965 call that new nurses would need a BSN to be able to sit for the NCLEX -- clearly that call on 'entry to practice' never came to fruition).
IMO, with 100+ schools enrolling students in the DNP degree, I think a achange will happen at some time in the future. When I counsel students, I ask them how long they intend to practice. If you intend to be an advanced practice nurse beyond 2020 (or so), I would encourage you to seriously consider a DNP program. There are two primary reasons for this: (1) pretend you are a patient looking for a primary care NP. Person 1 has a MSN degree, Person 2 has a doctoral degree and has published in his/her field. Who would you select as your practitioner. (2) You are the hiring official for your agency. You have two finalists -- one with a MSN, and one with a DNP. Who might you hire? Because of these two market forces, I think that APNs who practice beyond 2020 or so would seriously want to consider the higher degree.
There are not many BSN-DNP programs today. But there are more each year. Assuming you want to eventually get the DNP, you can decide to go for the full BSN-DNP program now (3-4 years fulltime), or do it in two steps (2 yrs to MSN, 2 more years to DNP).