I would suggest getting it done. I changed mine myself and was pretty easy (did all other belts and gave it a good service at the same time)
I too took the top cover off and it looked fine (I had a cambelt go on another car and it cost £2000!) so I thought I'd replace it plus I like getting my hands dirty!
I was very lucky as when I changed it (remember it looked fine under the top cover) there was a 10cm section of the belt that had serious wear, like nearly a 1/3 the belt had gone! my car too was at 80,000.
And when I had my old car rebuilt they gave me all the bits that come out and the cambelt looked nearly new, could read all writing on the belt! so dont be fooled by looks, I cant see lexus replacing the belt at 40,000 its not due then and any garage that did a cambelt would make a note in the service book.
If you like working on cars there is a great guild here http://www.lexusownersclub.co.uk/forum/index.php?showtopic=53085