Is Dragon Compendium Allowed? Yes, but individual issues of Dragon Magazine are not.
What about 3.0 materials? 3.0 materials, whether online or in printed form, are allowed unless they've been officially updated to a 3.5 edition.
Are Dragonlance, Ravenloft, Planescape, Dark Sun, or Kingdoms of Kalamar allowable sources? The Dragonlance Campaign Setting is allowed, but the subsequent books for Dragonlance are considered 3rd party, and are therefore not eligible, despite the "WotC approved" status of those books. The same holds for Oriental Adventures (1st party) and the subsequent Rokugan books (3rd party). Similarly, materials from Ravenloft, Planescape, Dark Sun, and Kingdoms of Kalamar are considered 3rd party for purposes of this contest, and are therefore not allowed.
What about online sources in general? If the online source is a) published by WotC, and b) not replaced by an updated version at a later time, it is eligible. Use it, link it.
Where's the line drawn with "acceptable/unacceptable" for Unearthed Arcana? This will likely vary a bit from Chairman to Chairman. Item Familiars and Gestalt have always been verboten, since before IC migrated to GitP; don't expect that to change. Flaws have similarly always been noted as warranting a deduction; while I am Chairman, I'm extending that to Traits, though they warrant 1/2 the penalty in Elegance that a Flaw would because they're roughly 1/2 as useful. Alternate spell systems, alternate skill systems and alternate crafting rules all create an uneven playing field, and as such, will be disallowed for as long as I am Chairman. Bloodlines are ripe for abuse, and will be strongly discouraged as long as I am Chairman. Note that judges are allowed to look askance at any use of Unearthed Arcana not specifically mentioned above, at their discretion, and otherwise penalize Elegance according to their preference.
What, exactly, does the ban on Leadership mean? As folks have started to try to work around the edges of this one, I'm forced to spell it out more plainly. No Leadership, Draconic Cohort, or Feats that grant a similar ability are allowed EXCEPT Wild Cohort while Amphetryon is chairman. Any PrC you choose with Leadership or a Leadership-analog has that ability entirely ignored for this contest, as it may neither be used nor traded away via any means whatsoever.
What's the minimum score in a category? Assuming an entry is legal, the minimum score in any category is 1. If a judge is convinced that an entry is illegal by the RAW, the judge may give a 0 or decline to score a given entry. Because this contest focuses on Player Characters, an entry that is not technically allowed for a PC, but is viable as an NPC, counts as a legal entry, but may receive a minimum score at the judges' discretion.
Does the Urban Tracking Feat count for Justiciar's prerequisites? Not unless someone can link to an official WotC ruling saying it does.