How should I inform my advisor about struggles with Aspergers syndrome? Although similar questions have been asked before, I hope this is not a duplicate as I'm not asking about whether I should inform my advisor (that's already decided), but how and when to do it. 

My situation is a little different than most in that I'm a biology major, and hoping to study autistic spectrum disorders for grad school. Aspergers almost became my "special interest" in a way, and so I'm trying to incorporate that into my work. Under these circumstances, I would expect my advisor to be a little more sympathetic to my condition. However, because I'm not officially her grad student yet (doing a summer project before lab rotations in her lab, with the possibility to become a permanent student made clear), I'm not sure exactly when to tell her. Specifically, should I do it before or after officially joining the lab? I'm leaning towards before since if any problems arise because of this, I'd like to know beforehand and seek out other advisors. But I'm wondering if this could also create an awkward situation since she can't legally discriminate me?

Also, how much should I disclose about my struggles? Should I be matter of fact and just state that I have this disorder (assuming she understands the implications), or actually outline some of my challenges.
Most scientific places (at least in the STEM) are nerdy, or at least, nerd-tolerant. I have yet to see a physics or mathematics department where mind-reading or maintaining eye contact are required.

The contact with your (potential) advisor is always a tricky thing, so it depends much more on your common ground than your individual personality (for some advisors you may be better than a neurotypical). I wouldn't be afraid that she rejects you only because you say that you have Asperger syndrome.

But, to be honest, most people do not distinguish lack of social skills from rudeness, insensitivity, ignorance or showing off - even if they are told beforehand. 

If you want to tell your advisor that you have Asperger syndrome (or to be precise, high-functioning autism), be sure to explain how does it impact your needs (and in which other you can cope well). Letting her guess won't do any good (either she will ignore it (~= not telling at all) or take wrong cues (especially as the spectrum is quite broad, and coping skills vary a lot)).

The sound-sensitivity thing may be the hardest part - it depends a lot on the place (both in term of the building and culture). But even there you might get some adjustments (e.g. being put in a room with reasonably quiet lab members).

Before getting admitted, I would ask about adjustments you need (e.g. it is possible that there is no quiet enough place for you). I wouldn't bother telling about any personal struggles (unless she is your friend or she asks and you want to tell). 

How to start? It depends on her. But since her topic is autism, it should be always on topic, so you may start with "As you may have noticed, I do exhibit autistic traits. (Actually, I am diagnosed with ...). [the content ]". Use any moment she is not busy.

Source: I am into the topic (see links and collected research papers). I did my undergraduate course in a nerdy place, and my PhD in a nerd-tolerant place.

If you want to read a success story, I really recommend The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius  by Graham Farmelo (see a nice review/summary).