One of the new features available in TFS 2008 SP1 is that a TSWA URL can be configured in the server so that checkin notification emails will have links to the Team System Web Access page for the changeset, which allows you to see the diffs for each of the changes.
You can find the documentation for this feature at How to: Configure Work Item Hyperlinks to use Team System Web Access. You must also install the following fix (QFE) in order to use the feature: KB957196 – Checkin event e-mail alert notification doesn’t work (download). Without the fix, turning on the new feature will result in checkin notification emails not working at all.
You may be wondering why work item tracking email notifications do not also use the TSWA URL. This unfortunately fell through the cracks. The four simple manual steps that you can take to enable this are documented at Links for Team System Web Access 2008 Power Tool are enabled only for check-in notification e-mail.
If you are curious about how to construct your own links to TSWA pages, you can find documentation here.
[UPDATE 10/21/08] If you want to remove the TSWA URL from registration, you can run the following command after installing the hotfix (the RTM version of the command does not support removing the TSWA URL).
tfsadminutil configureconnections /tswauri: