Yes, it's an old request (by miillions of designers). So why can't you simply make the text box 'editable' and then curve the text/sentence accordingly, by reshaping the bounding box around that text? This approach seems like a logical path. If Keynote is suppose to be a superior/simply design tool, and not just another basic and boring presentation tool like PowerPoint, why not make this update?
Final Cut Pro X, Mac OS X (10.7.3)
The Keynote product team has the last word in functionality that is planned, implemented, and omitted. As fellow users, we can't comment on the product team's decisions, but you certainly can provide feedback from the application's Provide Keynote Feedback menu item.
Until that time when Apple rewrites Keynote, you will need an external vector graphics application, be that free (e.g. GIMP), or fee (Affinity Designer, Pixelmator, etc.) that allows you to flow text on a path, export as vector image, and drop into Keynote.
to post feedback to Apple use the Keynote menu item "Keynote > Provide Keynote Feedback"
curving text in Keynote
