Karl Magnus Troedsson has evidently said that mod tools are a possibility still for BF3.  He said they definitely won't be in at launch, but they may possibly be implemented later.  Here's the quote:
 

“I will say that there’s been a lot of cool things coming out of the mod community for many years; I have a lot of respect for the people in the mod community, at the same time, as developers of a game of this magnitude — I mean, it’s the biggest thing we’ve ever done — we have to select what it is we’re going to spend our focus on. We cannot do everything; it’s basically like that.”

“We have heard the community very loud and clear on this topic — that they want mod tools, we are considering it, back in the studio. The game won’t ship with mod tools, but we have heard it. I’m not saying that we’re going to do it, I’m saying that we are thinking about it.”

They released mod tools for Black Ops, Battlefield 3 has no excuse to not have them. Sometimes they argue that the engine is too complex, but. regardless of how advanced Frostbite 3 is, the users themselves have proved more than capable of handling complex mod tools.

They've already stated that their engine is placed all over server farms, so I can't imagine that it'd be easy for them to package it into a manageable tool. It comes down to "makes it easy for us" or "make it possible for everyone else to use too" and I guess they just went with the former. If anyone thinks they're entitled to mod tools, then go die. I know it's a shame that BF3 probably won't have them, but go die.