I admittedly haven't been there in about 9 months though and I never noticed a homeless problem while I was there. Everyone seemed to be a contributor. Have things changed?
Being on Mission St., it seems there's always been somewhat of a problem of Mission St. craziness colliding with Noisebridge awesomeness. However, from what I can see not being in the space a ton (I live in Palo Alto, and only make it up a couple times per month at best), there's been added pressure on the space in the past 6 months or so. Anecdotally, it seems word has gotten out in the homeless shelter community and Occupy community that Noisebridge is an awesome place to store your food/stuff, hang out on the free wifi, and sleep on the couches. Needless to say, that's not seen as excellent use of the space, and has been a challenge for the volunteer run, often introverted, consensus based organization that Noisebridge is.
Having said that, I've been up there several times recently to hack on hardware projects, and Noisebridge has been awesome, vibrant, and filled with great people working on cool stuff. It's just going through some challenges that would benefit from people helping promote it in the right light.