It's for Source, as I said (Half-Life 2). It's a very flexible engine that can sustain an extremely large map (larger than Tribes), is technologically and graphically advanced, constantly under improvement, has a huge number of people that already own a copy, and most importantly for this crowd--scales very well across all grades of hardware.
Unlike any particular Battlefield game, the Source engine is continually improved. It's been in use since 2004 and will likely see service until beyond Valve's current line-up of projects. That means there is a lot of game time for any mod. Since anybody with any Source powered game (CS, HL2, HL2 Ep1, DOD, etc) can play a Source mod, there are a lot of potential users already. The game is also more combat friendly and less laggy than BF2.
So, that's why I chose the Source engine for the project. As for progress, it needs to be fully drawn out before any serious development can take place. Like I said earlier, I can never seem to find anybody that will actually work on the project. Until I can get some people who will actually stick around and work on it (heck, plan out what direction to take even) there isn't a whole lot for me to do. I can't do it alone, and I don't want to make all the gameplay decisions by myself. Afterall, I'm pretty out of touch with the whole RPG genre when it comes to playing. I need some people who know what would be maintainable and most of all--fun instead of repetative and boring.
If you'd like to join in, all you need is a copy of HL2.