Don't know about the RX but on the MK1 IS If you lose your transponder key or "masterkey" then it's new locks, new ignition barrel, new ECU and new keys. This must be common as Lexus do it for a fixed price of £2000! :(
Dies not seem to afflict the Mark 2 - produce any key to get a new key from scratch.
That's good to hear as I want a mk2 next year. I guess Lexus learnt from their mistake :)