I have, in the end, found the issue: a conflict with my font manager: Linotype's Font Explorer X. Linotype have now issued an upgrade—to version 1.2.3—and, if you use Font Explorer, then I highly recommend that you download the upgrade.
Bug fixes (1.2.3):
System font requests for auto activation caused requesting applications to hang until FontExplorer X is quit.
Downloading the upgrade will enable applications to launch as expected.
On another software note, do not install the Mac OS X Plaxo Toolbar. It is a pain in the arse and, further, it is near-impossible to get rid of. It's uninstaller package does not work and I am now having to hunt through my system and manually delete all of the attendant files. Because of this, I consider the Plaxo Toolbar for OS X to be malware.
Suffice to say, yesterday was not only very unproductive, but deeply fucking irritating. Apart from the evening, which was spent in the company of some pints and an old friend and was really very pleasant: 'twas an excellent end to what had otherwise been an utter bitch of a day.
Anyway I am now back up and running and posting shall resume presently...