Thanks to Steve Perisho of Seattle Pacific University for this. Cambridge University Press will be republishing the whole 61 volume run of the Blackfriars edition of the Summa theologiae in paperback form this February (2007). This handy translation, begun in the 1960s and completed in the 1970s, had the singular merit of being a facing-page translation, with the Latin text of the Summa on the left-hand page, and the English translation being on the right-hand side. The division of the work into 61 volumes also meant each volume was compact and portable. It also sported interpretive notes at the bottom of the pages, as well as appendices of articles that helped one situate a given tractate in its doctrinal or historical context.

The only "issue" with the translations was that the whole series was done by many people (all skilled, of course). But if the Italian adage holds—traduttore traditore (the translator also betrays)—then the variety of hidden interpretations in the series is multiplied exponentially. That said, one always has recourse to the Latin text on the opposite page. Here's the blurb from the CUP website:

Summa Theologiae. The complete paperback set
60 volumes, plus one index volume

The Summa Theologiae ranks among the greatest documents of the Christian Church, and is a landmark of medieval western thought. It is regularly consulted by scholars of theology, philosophy and a range of related academic disciplines. This paperback reissue of the classic Latin/English edition first published by the English Dominicans in the 1960s and 1970s has been undertaken in response to regular requests from around the world. The original text is unchanged, except for the correction of a small number of typographical errors. The parallel English and Latin texts can be used successfully by anybody with a basic knowledge of Latin, while the presence of the Latin text allowed the translators a degree of freedom in adapting their English version for modern readers. Each volume contains a glossary of technical terms and is designed to be complete in itself to serve for private study or as a course text.

The cost of the whole series is $1800.00 USD, far above the purchase-range for individual scholars, but a good investment for college and university libraries. I cannot tell from the website whether individual volumes can be purchased. I'll look into this, and post a follow-up.