Sorry to dig up the dead, but DO NOT count on an IS430 making production! It's simple, really. There's a GS430 that can be had for about $50-55K, and there's no way that Lexus would allow that to take away sales from the GS430.
However, there's a funny side note to this. Upon reading an old Car & Driver issue from early '99, there was mention that the "Toyota Altezza" will make it to the US with the 3.0-liter I-6 engine that powers the GS300. At the time, also Lexus offered the GS400 (a V8), which doesn't even take up much more space than the I-6. C&D noted "Note: an engine swap is thus quite possible...", which definately fueled the fire for a possible IS400/IS430 before the product hit the showrooms.
On the other hand, we have ahad a few customers inquire about an professionally-installed "IS430". The cost? About $15,000-17,000 when it's all said and done. Expect your warranty to be fully null and void at the end of it all.
Lexus has no production plans for the IS430 in the near future, thay don't sell that many GS430's compared to GS300's, and the IS300 is a decent, if not chart-topping selller. If any changes occur, they may come in the form of a IS330 (using the V6 3300 VVt-i found in the ES330 and RX330), but more likely to come form the upcoming '06 GS sedan and a new engine that will probably come with that car.
Over and out.