As detailed by Sean Nolan on FamilyHealthGuy – HealthVault now has

1. A Java wrapper library on available now on codeplex under Microsoft Public License.
2. Complete .NET SDK Source tobe released in Spring 08 under Microsoft Reference License.
3. Commited for Open Specification Promise process to be launched in Fall 08.

What this means is -

1. You can work with HealthVault using non Microsoft Technologies
2. You can refer to the .NET SDK if you want to develop a wrapper library to work with your technologies.
3. You can implement healthvault yourself!! – so no lock-in!
4. Transparent and Comminity driven – the process by which healthvault gets developed is transparent and open to participation by the community.