There is a short note in the SP6 Upgrade notes about improving the startup performance of BES.

I wondered if any of the larger environments had decided to deploy the Reg Keys and if they have, are the users whose devices don't receive the Service Books for a couple hours, still able work as normal?

By normal I mean bi-directional PIM Syncs, ability to browse, etc, etc.

Below is the note from the SP6 Upgrade Doc.

Improve the startup performance of the BlackBerry Enterprise Server

After an upgrade, the BlackBerry® Enterprise Server might send new service books to every BlackBerry device. If it sends all
the new service books at the same time, the BlackBerry Enterprise Server startup time might be affected. You can improve
the startup performance of the BlackBerry Enterprise Server by configuring how many service books the BlackBerry Enterprise
Server sends each hour.

1. On the computer that hosts an upgraded BlackBerry Policy Service, open the Registry Editor.
2. In the left pane, navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Research In Motion\BlackBerry Enterprise Server\BlackBerry IT Admin Server.
3. Create the following DWORD values:
• SBThrottlingEnabled: Set to 1.
• SBThrottleLimit: Set, in decimal format, the maximum number of service books that you want the BlackBerry Enterprise Server to send each hour. The default number is .60, which means the BlackBerry Enterprise Server sends 60 new service books each hour.

thanks!