Over the weekend, BioWare let slip some information about Mass Effect 3 for the PC, but it may not be what you wanted to hear. Mass Effect 3, both physical and digital copies, will require Origin to play on the PC and will not appear on Steam. There will be a one time authorization for the single player campaign, while the multiplayer will requre an Internet connection at all times. The good news is there is no limit on the number of installs for Mass Effect 3, just you will need to have Origin installed on your computer if it is not already. As for the lack of Steam availability, BioWare echoed EA's sentiments about Steam's "restrictive terms" that limit developer access to the customers. BioWare, and EA, are happy to partner with other downloadable services that let the company release patches and DLC directly to the customers, but will not deal with Steam.
BioWare also said that Origin is not spyware, and in order for games and patches to install, implements a Windows permission change to review the folder Origin is installed in. This is a normal Windows function and does not gather any information, despite what you may have heard. You will be able to opt-out of information gathering inside Mass Effect 3, so you should not have any issues.
Mass Effect 3 will launch on March 6th for the PC, PS3, and 360. A demo should be arriving shortly, and will presumably be available on Origin.