Does the auto box in the IS 250 'learn' your driving style only during the first 1000 miles?

I hope not! I'd expect it to be continuously adjusting itself based on how you're driving.

Hopefully it's not as bad at learning as the old RX box! Just see the thread on "Lurching" on that forum

Has anyone got a definite answer on this?

I mean if you sold it the new owner would have to get the ECU reset if it does stop learning after 1000 miles

I can't point you at any definitive statement, but a system that stopped learning would be of very little use. My wife drives differently to me and when driving in Cumbria the gear box tends to shift up later than when I'm driving in Cambridgeshire.

It's a continuous thing I'm sure. It's not confined to Lexus - my old Honda was the same.