I'm going to try my hand at this:
Comics that remain unomnibussed (after January 2014) are:
*Dawn of the Jedi 0-15
*KOTOR 36-50 +War 1-5 + Handbook (Handbook might be in one of the announced omnibuses; remaining issues will certainly be in volume 3)
*The Old Republic 1-6 + The Lost Suns 1-5
*Lost Tribe of the Sith: Spiral 1-5
*Knight Errant 0-15
*Jedi vs. Sith 1-6
*Jedi: The Dark Side 1-5
*The Clone Wars digests
*Clone Wars Adventures 1-10
*The Clone Wars Magazine comics
*The Clone Wars 1-12 + Free Comic Book Day 2009
*Darth Maul: Death Sentence 1-4
*Purge (original, Seconds to Die, The Hidden Blade, Tyrant's Test 1-2)
*Darth Vader and the... (Lost Command 1-5, Ghost Prison 1-5, Ninth Assassin 1-5)
*Dark Times 13-32 (it looks like Blue Harvest 0 will be in the Dark Times omni, but not the rest of that arc, I think)
*Agent of the Empire 1-10
*The Force Unleashed I and II
*Star Wars 1-onward
*Classic Star Wars 1-20 + The Early Adventures 1-9 + The Constancia Affair + The Kashyyyk Depths + Planet of Kadril (the old newstrips from the 70s and 80s)
*Jedi Academy: Leviathan 1-4
*Legacy 0-50 + War 1-6
*Legacy Volume 2 1-ongoing
*The Star Wars (not yet published)
*Star Wars Tales 1-24
*PhotoComics I-VI + Clone Wars
*Manga Episode IV-VI (four books each) + EPisode I (2 books) + Manga: Black and Manga: Silver
*Free Comic Book Day 2011, 2012, 2013
Note that the contents of Omnibus: Wild Space Volume 2 is fairly presumptive; I assume Wookieepedia has it right, but it isn't 100% official.
The Thrawn Trilogy, the Dark Empire Trilogy, and the Crimson Empire Trilogy hardcovers essentially count as omnibuses. I seem to recall Randy Stradley saying that those books wouldn't get the omnibus treatment.
The current list of comics at Wookieepedia (http://starwars.wikia.com/wiki/List_of_comics) is pretty accurate, I'd say. The things it does not include are:
1- Darth Vader and the Ninth Assassin 1-5
2- Legacy Volume 2 1-ongoing
3- Star Wars 1-ongoing (the new series)
4- The Star Wars 1-8 (the one based on the original drafts)
5- Dawn of the Jedi 11-15: Force War
I think that's it.
Note that the November 2013 solicitations (which included the announced Omnibus: Dark Time Volume 1 and Omnibus: Adventures for January 2014) is the first to include two omnibuses.
I've bought every omnibus there is, even when I own everything in it in issues or TPBs (omnibuses are easier, and it means I can give issues or TPBs away to family later). I'll be skipping Adventures though, just because I like the digest books as they are.