OSGi services provided by p2 and Update Manager are now registered using the OSGi declarative services mechanism. If you are writing your own application that makes use of these services, you need to make sure the declarative services bundle (org.eclipse.equinox.ds), is being started in your application.
In the 3.4 and 3.5 releases, p2 provided a provisional API for accessing its functionality programmatically. This provisional API has largely been replaced in the 3.6 release with a fully supported mature p2 API. While there is generally no migration support for clients who used provisional APIs, you can find some help with migration on the p2 Helios Migration Guide.