Environment-switching is now a feature in the HealthVault Connection Center user interface. For the old-timers out there -- you no longer need to modify any registry entries! You just need to be logged out of HealthVault Connection Center and look for the "Services" menu in the upper right corner:
Select "Developer" to point at https://platform.healthvault-ppe.com/platform or "Production" to point at https://platform.healthvault.com/platform.
In the past, you needed to install the HealthVault SDK in order to get this feature. That is no longer the case. You just need to install HVCC and you need to be logged out in order to see the "Services" menu.
(updated by Tremonte on 3 Feb 09, 4 Feb 09, 12 Nov 09)