Hi Johannes,
First, an aside: In terms of full integration, PixelSnap will always be an extension, so it will never fully "integrate" into Inkscape. (I'd like to write some fully integrated code for it at some point, but that's just a pipe-dream -- not going to happen any time soon).
However, the extension has been included in the main Inkscape source repository, so that it comes with Inkscape when you download Inkscape -- no need to download it separately from the PixelSnap website. Except...
...the version included with Inkscape is a bit outdated. At some point, I need to completely eliminate this, and commit all my latest code to the Inkscape repository, and delete the Google Code copy. However, I've been too busy to do this for a while, so I just haven't gotten to it.
So, to answer your question: it will happen eventually (say in the next few months). For now, I'd suggest continuing to follow the Google Code project.
I'll post a big clear message out to everyone when things change.
- Bryan