It is definitely serif.

One word of caution that I found out using my 300 and 900 with extra fonts. You can of course rename a non-DRMed ePub to name.zip, extract it, modify the files, rezip it, then rename it back to name.epub. But in some cases the Sony reader will not read these modified books. I'm not sure why and not all are unreadable, but some will generate errors and Sony Readers will puke.

To avoid this problem I would copy the epub and rename it to name.zip, extract it, and view the CSS file. But rather than modifying the copy I would use calibre to add the original epub book, edit the metadata if necessary, then convert it and add the CSS changes (overrides) during conversion. Sony Readers did not puke on these like they did the hand modified books.