Thank you for this! I was getting a "AdditionsVBoxService error: VbglR3Init failed with rc=VERR_FILE_NOT_FOUND" after upgrading from Debian 6 to 7. Installing the linux-headers for the new 686-pae kernel, reinstalling virtualbox-guest-utils, and then restarting resolved my problem.

Hopefully this info will help others who run into a similar problem.