So I ran into an issue installing Forefront Server that I can't find any information on in Google, but I was able to deduce the solution by analyzing the log files.
The immediate symptom is that Forefront installation fails at the step where it is installing MOM Reporting. The error code you get is just...
Process: C:\WINNT\system32\msiexec.exe Exit code: 1603
That error code is completely generic - it just states that msiexec dumped for some reason. You have to analyze the MOMReporting.log in the %ProgramFiles%\Microsoft Forefront\Client Security\Server\Logs to find the true cause. There are a variety of causes, mind you, with wonderful solutions on TechNet forums. However, I couldn't find anybody talking about the problem I ran into.
The specific error in the MOMReporting.log file that was blocking my installation was...
VerifyPreRequisites: Result of Pre-Req check: PreReq_Requirements_Not_Met . See PreReq report for details: \MOMPreReqReport.html. Error Code: 0x80004005.
WHAT pre-reqs? The Forefront pre-req checker told me I was good to go! Turns out that MOM Reporting checks several things that the Forefront installer does not. Run the MOMReporting.MSI by itself (hit next a few times, type in the reporting URLs) and you'll get the pre-req failed report. Read it. Now here's an area that got me stuck for a while. IGNORE the SQL Reporting Services pre-req failure. Apparently, according to this article, this is normal in SP1-rolled MOM installers and the set-up application works around it so all should be well as long as you don't run into any other pre-req failures.
So what was my problem?
ASP.NET is not enabled
Well that's odd, Reporting Services IS ASP.Net so how can this fail? Well, if you are running Reporting Services in SQL 2005, that's an ASP.Net 2.0 application and the pre-req checker is looking for ASP.Net 1.1! So make sure Microsoft.NET 1.1 is installed, registered with ASP_RegIIS -i and also enabled in the Web Service Extensions in IIS. All should be well now!
Another tip: I found out that ForeFront (actually the MOM Reporting part again) sometimes will barf when you use a named instance of SQL Reporting. This seems to be due (but I'm not 100% sure) to the $ (dollar sign) being in the URLs to access SQL reporting services. Now it may be coincidence, but I managed to clear up a failed install by simply creating new URLs in the Reporting Services Configuration tool and replacing the dollar sign with an underscore (_). Good luck!