Seems a few people have encountered a problem with SSRS 2008 R2 after installing SP2.
In my case the SSRS instance would not restart, so I checked Event Viewer.
There were no messages at all under the Application log, and all the system log stated was:
Timeout (30000 milliseconds) waiting for the SQL Server Reporting Services (Grumpy) service to connect.
I tried changing the account that SSRS was running under, but it would still not restart, and I was considering un-installing SP2 (KB 948109).
A bit more investigating though, and there didn't seem to actually be any error from SSRS, so I tried changing the service startup timeout value to 60 seconds.
This time SSRS started successfully (albeit slowly). It did require me to backup the encryption key file, but I think that is more likely to have been related to playing around with the account it was running under rather than the timeout change.
The post I found that details changing the timeout is as follows: