After a bit of searching, I found the culprit: Implement full page zoom in the WebKit browser. A quick check in Terminal...
defaults read com.apple.Safari | grep WebKitDebugFullPageZoomPreferenceKey
...confirmed that this value was set for Safari on the Mac Pro, and not on my MacBook pro. The solution was simple -- quit Safari and delete the troublesome key in Terminal:
defaults delete com.apple.Safari WebKitDebugFullPageZoomPreferenceKey
On relaunch, Safari once again respected the setting of the Zoom Text Only menu item. (This key causes the same problem in Safari in both 10.5 and 10.6, though I'm not sure if the key survives an upgrade from 10.5 to 10.6 -- my Mac Pro is still running 10.5.8.)

