Debugging Timer Jobs in MOSS 2007

In the Execute method of the job (the method you override to write your job execution logic) place the following code (Thanks Joe!): System.Diagnostics.Debugger.Break();
Assuming you have already added your solution to the site and deployed it using Central Admin, simply uninstall the job assembly from the GAC and add the newly built one back in again
What I did here is to stop the OWSTIMER service (to prevent my job from running a million times while I do these steps)
Remove and copy the job assembly to the GAC
Map the GAC folder so you can actually see the files to place the .pdb file in for debugging using subst Z C:\Windows\Assembly (this command run from the command prompt maps the file structure of the windows assembly folder to the z:\ drive so you can copy your assembly into the Z:\GAC_MSIL\MyAssembly\10000___StrongNameKey\ folder
Reset IIS and restart the OWSTIMER service
Quickly attach your VS.NET debugger to the OWSTIMER process and wait for your job to kick off inside OWSTIMER. The debugger will break on the System.Diagnostics.Debugger.Break(); allowing you to step through your job