Aptara just announced their latest
SaaS product,
eGen. It's a large scale Epub creation software aimed at major publishers.
I'm sure it's nice and all, but even with a large number of titles to convert I'd still stick with calibre. I have had a few conversion runs of two hundred or so titles. Calibre was more than capable of the task. And this was on my 1.8GHz Pentium M laptop, too.