I imagine you've seen this website: http://freepages.genealogy.rootsweb.ancestry.com/~sterth/
The site creator seems to have transcribed as many LDS microfilms as he could to the website. I think I'll see if I can verify the data myself but it's a great start.
My ancestor, Richard Penberthy, emigrated from St Erth to Pennsylvania in the 1840s; he was a coal miner. I'm descended from his daughter Mary Jane Penberthy. I'd love to visit St Erth someday if I'm in the U.K. ... it seems like a picturesque part of the island, and the font where our ancestors were baptized is still there in the church, as it has been for 700 years.