I have the same problem, appears that my daughter lost one of the keys to our Lexus 350.
I was researching online, it has the same issue, replacement key needs to be programmed with special equipment. It appears that there are locksmiths that can do it, and would be cheaper than the dealer. For my car, online said it was around $300 for a replacement key from the dealer. I will give them a call today, and also call some locksmiths to see if they can do it.
I found this place to get the keys from
http://www.keylessride.com/default.aspEDIT:
One local dealer wanted $330 for the key and $50 for programming. Said it would take 45 mins.
A local locksmith wants $230 for key and programming. They would come to me and do it.
Randy