i believe the "S" originally meant "saloon".

in "SL" form, it meant "Sportlich/Sportive" or in some cases, "Special"

In all cases, they are all "Spensive".

I believe the changes over time in nomenclature reflect the changes in influence between marketing and engineering within the company. Previously, it was engineering for engineering's sake, no matter what the market wanted, and the names obviously referred to technical designations that appealed to the technical-minded set. Nowadays, they are marketed in "classes", which evoke different lifestyles, or levels thereof. Hence the letters come before the numbers and some of them are given new meanings.

This could also be a response to the built-in class designations of the BMW nomenclature (3-series, 5-series, 7-series), each of which has distinct target markets.