To the best of my knowledge, it's the low speed traction control system (available in many mainstream cars today - yes, the Solara has the same system), that works off the line. I think up to 30 mph. So if you're going 60, and you experience tire slip, I don't think the system will work. That's where VSC (vehicle skid control) comes into the picture. Lexus will (most likely) offer VSC on the IS300 next year (2002 model). I suppose they don't wanna put everything in the first year...to keep sales steady, throughout the lifecycle, they keep adding features and options.
But the traction control on the IS works just fine. In fact, for sporty driving you have to turn it off or it keeps preventing wheelspin.
If the traction control in ES is the same as IS, then I don't think it would work up to 30mph. I had the traction on in my previous ES while merging the traffic on a highway and it was about 80Km/h or around 40mph ?