I honestly doubt that Terra has it's own dedicated PDF, simply because I have a hard time envisioning any form of real society on Terra that would support one. Who would you pull the from? The decadent nobles or the massive clerk-hordes of the Administratum?
The following is just my interpretation, but I'd figure that even Terra has a "normal" citizenry in that not everyone is a noble or a clerk. Whilst those occupations may be unusually bloated on this world compared to others, there's bound to be countless families whose members are merchants, public transport operators, law enforcement, construction workers, Ecclesiarchy clergy (including the Ecclesiarch himself) and their lay servants, repair guys, courtesans, ...
I mean, even if we assume that all production of foodstuffs and vehicles etc happens elsewhere, there are still so many jobs attached to just distributing these goods and keeping the buildings from collapsing, etc. And I'm fairly sure even Holy Terra has a vast underworld that is just as grim and decayed as Necromunda's. If we look at the Imperium of Man as being inspired by the Roman Empire, then we can also look at the city of Rome.
A statement like "all of Terra is clerks" kind of sounds like "Mars is populated only by Tech-Priests" - yet the Fabricator-General has his own Tech-Guard anyways.
Besides, if you want a standing military on Terra, you'd just draw them from the same source you draw the clerks from. The question is really just whether you want a PDF or not. And personally, I'm assuming the Codex fluff about the Imperial edict on planetary defences to apply to Earth, too.
Speaking of GW fluff, I also just remembered something from Inquisitor:
"The most powerful of the Navigator houses have their Family House on Terra, in a vast district known as the Navigator's Quarter, a byzantine labyrinth of ornate buildings, decorated beyond any measure of taste. Every palace is huge and decorated with mighty murals and elaborately painted ceilings, the Navigator Lords competing with each other to create the most beautiful palaces, adorned with the greatest works of art in the galaxy. They have libraries containing millions of books, data crystals and scrolls, and own collections of sacred relics to match anything possessed by the Ecclesiarchy. They own menageries of rare beasts, and wine cellars replete with the products of a million worlds and live in obscene luxury and splendour. [...]
Open warfare, in a controlled fashion, is not unknown between the feuding houses, since all control vast wealth, large professional mercenary forces are paid huge amounts to serve the Navigator Houses. Some of these mercenary contracts date back thousands of years. The Navigators, paranoid of their rivals, are afraid to be seen as militarily weak, and thus they pay well for loyal service, and failing this, they trade in slaves or penitents and train their own soldiery within their huge private estates. Slave trading is a lucrative business and so long as taxes and tithes are paid, the Administratum turns a blind eye."
So I guess we can add mercs to the list, even if they'd probably be a bit unreliable in that they would focus or even limit their attention to the property of the Navigator Houses. Then again, an emergency could see them drafted into the PDF by Imperial edict.
 Something I just remembered ...
Some time ago I theorised that the Storm Trooper regiment might originate from Terra and its special role in the Munitorum is a relic/tradition from before even the Great Crusade. If this were so, it might still have its base on Terra as well, and even though much like the Battle Sisters they'd be active throughout the galaxy and send entire companies away, there is a good chance to have several thousand Storm Troopers stand ready for immediate deployment, together with any new recruits from the Schola Progenium.
This is a bit useless now, since GW has recently retconned the Storm Trooper regiment from "one" into "lots", but maybe this idea can still serve as inspiration. And even hundreds or thousands of ST regiments still need to be headquartered somewhere important, right?
The Adeptus Custodes, as well, are to Space Marines what Space Marines are to man, each one of them an individual investment likely capable of destroying minor cities, given enough time to pound on things.
Never been a fan of those OTT high fantasy interpretations ... but that's a matter of taste, of course.
Edited by Lynata, 12 May 2014 - 02:33 PM.