According to a report in The Bookseller, over 200 romance titles are available.
At the Mills & Boon site, here are:
The eBooks home page
The eBook FAQ -- the eBooks are in ePub, but they also mention, confusingly, them being in "Adobe eReader" format. (Behind my back, Acrobat has now become the Acrobat eBook Reader!). Adobe Digital Editions is required. Further confusing things, they say the eBooks can be read on a PalmOS handheld! I'm not aware of any ePub readers for PalmOS. Making things even more confusing, the eBooks home page mentions reading them on a Blackberry!
All 221 (at time of post) available titles (scroll down)
Book browsing is done using a Shockwave interface from LibreDigital. It's pretty slick.
Aside from being sold at the Mills & Boon site, these are also available from the Waterstone's eBook store for the Sony Reader.