Paul Thurrott confirmed (via his reliable source) on Windows Weekly, which aired yesterday (Thursday 22 March) that:
1) AT&T will make the formal announcement on Monday 26 March 2012 about the Lumia 900 (no word on Titan II).
2) The device will be available Sunday 8 April 2012 (online, although Paul didn't confirm, we can safely assume)
3) The Lumia 900 will indeed cost $99 (subsidized), which cause Leo Laporte to go crazy over the price he paid for the Note ($299 subsidized) and says this is a big move for Windows Phone.
If you want to listen to that segment, go to
this link and skip forward to the 48:45 mark and they talk about the Lumia 900 for about 2 minutes.