SP1a is the same as SP1 EXCEPT it removes Microsoft's proprietary Java. SP1b will include Sun's JVM. If you want to make your machine like 1b, uninstall MS Java and download SUN's JVM. This assumes you have SP1.
Don't know when 1b is coming out - it might depend on the outcome of MS's appeal.
This whole thing has to do with the court fight between Microsoft and Sun Microsystems.