With 550+ aircraft of differing 737 variants in the overall fleet, the existing numbering system no longer had the needed flexibility with the addition of new -800s entering service. Under the old system, the -800s would have been N800xx, N801xx, etc. with "xx" being "SW" (but maybe not, since Skywest also uses "SW" registration suffixes). And what would happen the next -800 after N899xx? N900xx? Nope, since the 900-series numbers are used on some of the -700 fleet (along with the -700 series, -200 series, and -400 series).
A better system was needed, and the new systen is based on a 4-digit number, and a single-letter suffix versus the old 3-digit number and 2-letter suffix. This started with the new -800s, and they all use an 8000-series number:
http://www.airliners.net/photo/South...a025190b7c1b51
http://www.airliners.net/photo/South...a025190b7c1b51
With the -800s, you'll notice that some start with N80nnX and some start with N83nnX. The first of the four digits ("8") signify a 737-800, and the second digit ("0" or "3) signify whether the aircraft is a non-ETOPS -800, or an ETOPS-equipped -800. The third and fourth digits are just sequential numbers, and the single letter suffix is just a letter with no specific meaning.
As the other fleets go through heavy MX, they too will get re-registered, and the new scheme also allows easy identification of aircraft originally ordered by bw SWA, and those aircraft that SWA acquired from sources other than Boeing. Internally, aircraft will be identified ny the nose number, which will be the registration number, minus the single-letter suffix.