The Systems Center Operations Manager 2007 team blog has a great post on using SiteName when deploying gateways to help manage alerts in case you haven’t seen it. This one comes from Rob Kuehfus and he includes a high level overview as well as a link to the detailed steps:
When deploying the Operations Manager gateway role you can tag the gateway with a Site name. Any alerts coming from an agent reporting to this gateway will now have it's "Site" property populated with the site name you configured when deploying the gateway. This can be really handy for building specific alert views and sending notifications.
J.C. Hornbeck | Manageability Knowledge Engineer