It looks like as of midnight Eastern time all three sites posted updates and added Marvel to their libraries. Here's a breakdown of what each app is offering:
Amazing Spider-Man #1-25Amazing Spider-Man #519-523Astonishing X-Men #1-24Invincible Iron Man #1-16X-23 #1-6X-Men: Age Of Apocalypse #1-6
Astonishing X-Men #1–24Captain America #1–30Marvel Zombies #1–5X-23 #1–6X-Men: Age of Apocalypse #1–6
Amazing Spider-Man #519–524Astonishing X-Men #1–12Captain America #1–7Invincible Iron Man #1–6X-23 #1–6X-Men: Age of Apocalypse #1–6
iVerse and comiXology are offering Marvel's comics for $1.99. Panelfly has them for $.99 each for the first few issues, then $1.99 for later ones.
Update: CBR's Kiel Phegley is working on a piece for the main site now, and sent me this tidbit ... per Marvel, there's a fourth partner as well, ScrollMotion. There's no mention of it on their site yet, however. ScrollMotion has published a large number of books for the iPhone, including titles by Stephen King and Stephanie Meyer, among many others. If you do a search in iTunes for ScollMotion or Iceberg Reader, you can find all the books they've released.
Also, Marvel has posted more information on their website.