External monitor stopped working after 10.9.2 update
I've just managed to workaround the problem by disabling DisplayPort 1.2 within the Dell OSD. 

To do this, you might need to use a bit of a workaround, which is to get the monitor to bring up the input select and then hold down the green tick for ~8 seconds when the (mini-)DisplayPort option is selected. This will bring up a new menu that enables you to enable/disable DisplayPort 1.2 mode. (Green tick to enable/red to disable, etc.)

Whilst I suspect this is just a temporary fix, it at least means that I can use the monitor until Apple fix their seemingly problematic 10.9.2 release.

UPDATE

As an minor update, my bug report has been closed as a duplicate and due to the nature of Apple's "closed" bug tracking system I've no means of viewing the other ticket. However, at least this is a known issue that will presumably be addressed in a successive release.