Nice article - you seem to have laid out a good foundation of topics to cover. There are just a couple of points I wanted to add:
The SQL Server-native geography and geometry datatypes are NOT stored using Well-Known Binary format. WKB is administered by the OGC, just like WKT and GML are. Internally, SQL Server uses a custom binary format.
Although the WKT representation of a geometry/geography instance may be defined using a z coordinate (representing the elevation/depth of a point), none of the inbuilt methods actually perform any operations using this coordinate, so it's for storage only. All operations take place on the (flat) 2-dimensional plane of the geometry datatype, or the (curved) ellipsoidal surface of the geography datatype.
You say that geographic coordinates of latitude/longitude are most common, but that depends a lot on the sort of data you want to use - for instance, here in the UK most data uses the GB national grid system (and many other European countries use their own national grid), and most of the detailed spatial information for the US is provided at state-level using the relevant state plane coordinate system (such as from http://www.mass.gov/mgis/database.htm). If you want to use this sort of data, you need the geometry datatype, not geography.
Other than that - good stuff! I look forward to the rest of the series.
For anybody interested in finding out more about spatial in SQL Server 2008, check out:
"Beginning Spatial with SQL Server 2008" (Apress)