[LX5liter]

I agree with the bad bearing diagnosis. Remove the belt and turn the accessories by hand and you should be able to feel the roughness in a bad bearing. All the accessories should turn fairly freely, with the exception of the power steering pump which will be stiffer. What gtsr515 means is get a shorter belt and bypass the smog pump to see if it still squeals on start up.
For what its worth I have also seen ( heard ) alternators do this on start up, so check out all the accessories.