User blog comment:Aleksandr the Great/Prothean Squadmate is Officially DLC/@comment-1221773-20120222232942/@comment-1221773-20120223034632

BioWare claims the DLC was worked on in January. This jives with both the audio files (which were pulled from the demo, which very likely was put together at the same time) and the Prothean's arm appearing in that multiplayer trailer (which was released in February). As for the Prothean appearing in the leaked script, it's obvious that the character was planned for long before the actual DLC was developed. We did hear about him when the CE was first announced after all. I bet there was some work done on the DLC over the course of ME3's development, but it was never finished until after ME3 went into certification. You know, not everything is some evil conspiracy to rob the masses of their money.

And Ironreaper, as I explained to Aleksandr on his talk page (beware of spoilers) I'm pretty sure the Prothean doesn't play as significant a role in the game's storyline as you may think.