The decision in Heller doesn't really change a whole lot from a practical point of view. It basically endorses the status quo: that the government has the right to reasonably regulate ownership, possession, and use of firearms. This has been pretty much the standard interpretation of the second amendment among the lower courts, absent further guidance from the Supremes. Heller draws some lines around what constitutes "reasonable regulation", but for the most part, not all that much has changed.
From a rights-based standpoint, it's a very curious decision, because Justice Scalia has basically invented a new constitutional right out of whole cloth--the constitutional right to self-defense with a firearm (there has always been a common-law right of self-defense, the exact parameters of which vary by jurisdiction). It is, to my mind, much more of a stretch to find a constitutional right to self-defense with a firearm in the depths of the second amendment than it is to find a right to privacy in the fourth amendment. As others have noted, this is pure judicial activism, no ifs, ands, or buts about it.
I personally think that the opinion got it badly wrong and bends himself into pretzel knots trying to explain away the militia preamble.
There's also the question of whether or not the second amendment applies to the states by operation of the fourteenth amendment. I would think that it does, though I'm not one of the nine people who are qualified to make a definitive statement on that subject.
There will be, of course, considerable litigation about where exactly do the lines of 'reasonable regulation' fall.