SBS 2000 had a form of monitoring it it, but it wasn't that useful. SBS 2003 took Monitoring to the next level to really try to help remote administrators, or local administrators for that matter, monitor the server's health without having to look around all over the place to find it.
A little history here that may or may not be interesting to you. The monitoring feature came about way back when the Small Business Server product group started using our own server. One of our test leads was in charge of managing the server. This being our first attempt at Dogfooding Small Business Server, he was interested to make sure the server was running every day.
Naturally this turned out to be a pain and Monitoring was taken to the next level.
The SBS Monitoring uses a combination of products, including Microsoft Health Monitor (HealthMon as we call it) and Microsoft Database Engine (MSDE). HealthMon was used to capture events, and triggers, while the database is used to store this data so you can see how the server is doing over time. No on really wants to see how healthy a server is doing at 6am in the morning, it probably has minimal load. What you really want to see is how it's doing over a course of the day, what the highest process usage is. MSDE provides the storage capability to show these things over time.
A TechNet webchat on Monitoring Features in SBS was done Janurary of 2004 that explains more of how it works.