A few questions, pure curiosity....

Why not use WSUS to deploy the service packs and CU for SQL Server? How does your shop handle the OS updates if you do not have WSUS in place?

Why reboot prior to starting the SP install? I would imagine that is the easiest way to get all connections closed into the SQL Server instance, but that adds on to downtime for patching.

How do you handle errors/issues if the SP install breaks? I did not notice any thing in your batch file examples that catch if the SP installation errors and stops. If the SP errors your sequence would continue wouldn't it? My concern would be the additional reboots and the CU patch would continue and attempt to install itself.

A suggestion to improve your process would be to put this in PowerShell. I'm not saying that batch files aren't still useful, but they are outdated 😀 Especially with use of Window Server 2008 R2 and SQL Server 2008 you have added power in controling the environment and can do more checks and balances with each step of your processes. You could still do some with SQL Server 2005 and Window Server 2003 if you have PowerShell installed/enabled. WMI calls can get you the version and service state of SQL 2005 very quickly. This would also allow you to automate checking the bootstrape log and verifying the build number of the instance before starting each installation, also giving you a snapshot of it before and after.

The ability to do remote calls with PowerShell could also be a benefit in having one monitoring server do this, which would allow a central location to check status of all your installations.