Are the LEGO Friends Mini-dolls compatible with Minifigures?
In some ways but not others, according to this post by Catsy at The Brothers Brick:


  
  It was near-impossible to pull the legs off–I sawed through them where they join above the knees.
  The tab that connects the waist to the torso is completely incompatible with any standard System connection I’ve tried.
  The hands are not angled forward the way a minifig’s are–so accessories with a pronounced rake to them may not look as expected.
  The lack of wrist articulation is extremely limiting in terms of how you can pose them with accessories.
  The stud connection point on the feet is in the front, under the doll’s center of gravity and more or less directly under the body. The feet are slightly oblong.
  The legs have a very slight backward sweep on the way down, which you can see most clearly on the right leg above. The upshot of this is that it is impossible for a mini-doll to stand on any 1×2 area that has anything immediately behind them.
  The neck is a standard 3mm bar connection rather than stud-width like a minifig neck–the heads are incompatible with minifig torsos.
  The head is approximately the same dimensions at the top as a minifig head, but tapers towards the chin in a roughly egg-like way.
  The nose causes complications with some fully-enclosed headwear, but not most. The chin extends lower than a minifig’s chin, so that headwear with “chin straps” obscures the mouth.
  The hair is interchangeable with minifigs–and many TLC minifig hairpieces look quite good on the girls... it is made out of the same kind of soft plastic as the Exo-Force hair, and has tiny holes on the top and side of the hairpiece that go all the way through, allowing the attachment of hair accessories.
  Brickarms helmets work extremely well and look great. I do NOT recommend trying to use aftermarket hairpieces, however–I tried putting a third-party hairpiece on one of them and had to use pliers to get the head back out.
  


Note also that the legs do not move independently of each other - both left and right legs are on the same hinge. Not really a compatibility issue, but it greatly restricts the poses available without modification.

From the official press pack: