I haven't tested the NEX5, but being an APS-C DSLR user (D300), I want to comment on high ISO capability:
In the recent weeks, I spent several evenings on the Munich Oktoberfest doing night-time street photography. I set my camera to Auto-ISO with a min. shutter speed to easily allow hand-held photography (1/160s for EFOV of 28mm and 35mm). And I selected a rather large aperture (f4) because I wanted to see what kind of a performance I could get out of my camera's AF system (more about that later).
I had intended to shoot for BW conversion - so I set the camera'a auto ISO mode in such a way that it would use its capabilities right to its design limits. It turned out that my camera worked fairly conservatively: It would only increase ISO if it had reached the specified min. shutter speed. Nevertheless, the camera had to go up to 3200 and 6400 ISO in about 30% of cases (it was really dark at times).
I routinely converted my results into BW, and in BW, I think the pictures were perfect even at ISO 6400 (see examples in my gallery)! Mind you, DR will decrease, so exposure accuracy becomes more important, and decreases in resolution actually become visible starting at 1600 ISO (film is no better), but noise never became a problem in BW.
For curiosity, I looked into the color versions, and after having learnt more about film and its capabilities, I must say that even the IS0 3200 image files compare favorably to what I would expect from pushed color negative film!
But there's another benefit of modern digital technology, and that's autofocus (phase AF in my case). The trick is the capability to focus on the nearest object, and to use face recognition - plus dynamically following moving objects. It actually works. I got amazing results - I had an AF a hit rate that was better than guesstimating manual focus, with interesting OOF rendering because I could effectively use much wider apertures (I shot most of my Oktoberfest series at f4.0. IMO, zone focus only makes sense for these focal lengths if you work at f5.6 and above.). Any wider apertures are problematic because low-light AF accuracy still isn't perfect (but it's good enough in most cases).
What does that mean for my assessment of X100 performance: If that camera's auto ISO performance is similar, then it will be a very useable street camera.
As to AF under low-light conditions, I'm not so sure because contrast-detection AF inherently will be slower and has more tendency to hunt. Thus, I guess this will mean a more intense use of zone focussing. That won't be too much of a limiting factor because if auto ISO mode reaches a similar IQ as I get with my D300, we are indeed looking into a bright future with the X100.
