My darling Will,
Auntie may not be super up-to-date on the whole electronic publication standards thing, but she does a pretty mean Google when prompted by her many nephews and nieces. It wasn't hard to find some excellent (and free!) conversion tools.
The open source calibre ebook project offers a multi-platform conversion solution that supports PDF-to-ePub translation, among many, many other formats. The donation-supported project is about 4 years old. The application itself is not for the faint hearted, although honestly it's a pretty decent interface. It offers quite a lot of options and was obviously designed by engineers. I had no problem using it this morning to convert a handful of documents. Your mileage and comfort may vary.
While limited in terms of document size and absolutely bare bones in terms of interface design, the epub2go website offers a handy alternative. It easily converted several PDFs this morning for me into serviceable ePub documents. You browse to a file, upload it, and wait for the converted file to download. Meant for use with the iPhone, the site offers the option to e-mail the document (for use with the free iPhone Stanza application) or to download the converted document directly to your computer.
Although OS X makes it easy to print files directly to PDF from any print dialog, you cannot yet print to ePub format. These tools certainly will help. For those of you on Windows, check out bullzip and CutePDF, which add much the same print-to-PDF feature to any Microsoft Windows application.
Thanks, Corey Gilmore, Wes Cook, and everyone else who tweeted suggestions.