I've seen this a couple of times already, enough that I thought we should have some quick content on it. When you attempt to install the standalone package for Service Pack 1 for Windows 7 you might get the following error:
0x800f081f <--This maps to CBS_E_SOURCE_MISSING
You'll be prompted to run the CheckSUR utility and running that utility will not resolve the issue.
This may be due to a pre-release version of the RSAT tools being installed on the system. To alleviate this failure, do the following:
1. Uninstall the pre-release RSAT tools
2. Reboot the system
3. Install the release version of the RSAT tools (http://www.microsoft.com/downloads/en/details.aspx?FamilyID=7d2f6ad7-656b-4313-a005-4e344e43997d&displaylang=en)
4. Reinstall SP1.
Remember, if you want to utilize the RSAT tools with SP1 installations you must install them prior to upgrade until the repackaged RSAT tools are released in April.
If you have a Windows 2008 R2 Server installation that is getting the 0x800f081f error code then you are not hitting the RSAT tools issue. You have a source file missing somewhere that is causing the issue. I would recommend running the latest version of CheckSUR against the installation first to see if you have a missing or corrupted source file of some sort first. Otherwise, we'd need to look over your logs to see what else is going on. You can either open a post on the newsgroups or an issue with Microsoft support if you need assistance.