Hi Shalrocks:
The notification channels from Essentials 2007 are a potential 'weak link' in letting you know of a problem. For example, the SMTP email alerting does depend on the SMTP server being online. If that server you specify is down, you won't get the alert.
Essentials 2007 SP1 has an additional feature that makes the E-mail notification channel more robust, specifically, you can specify multiple, fail-over SMTP servers.
As for the Instant Messaging (SIP) and Short Notification Service (SMS) notification channels, there are no redundant service features.
Good luck,
John Joyner
MVP-OM