Ian,
Believe it or not I got spidered again by google, no index, no anything when I search google for me site, but thats besides the point.
I noticed that while they were re-spidering me, they lost the session Id's only when viewing the product_info pages. When the bot hit a category page and transferred to the cpath, they picked up the Sid again. Granted, there is no info on my cpath area that would make good content for an index, but it caused them to loop around again.
Just some input as to what is going on. Like i said though, they are losing the sid's on product_info.php but as far as I can tell, this is the only page they lose it on.