I've done it. My application to Cambridge has been submitted online. For better or worse, I clicked 'submit' and paid the application fee. There is no turning back now.
Of course, this is only really the first step on a long road that will culminate in either success or a rejection. After this, I have to send multiple copies of my supporting documents and references to the BoGS, and a separate copy of my entire application to the Dept. itself. Still, it is a small success for me, who as of this morning thought that the entire situation was hopeless and that I would never come up with a suitable response as to why I wanted to pursue a master's degree. (That said, I am not entirely sure that what I ended up with was a suitable response -- only time will tell!).
Now I wait until they tell me what else they want me to send. Back to gathering sources for my SMP (which is a whole separate issue in itself and provides enough fodder for a completely separate post) and attempting to sleep more than four hours a night.