So this article in the national old people newspaper, the NYTIMES, sorta dealt with this issue. One girl at NYU started a program to "rescue" abandoned bikes and give them to people, sorta like a social service. It elaborates a bit on the legal issues involved, and perhaps it can give some guidance on the moral issues as well. Here's the
link-up.
However, I do have to say, that just because you want to do something doesn't make it morally correct. This may seem obvious, but it shows up more in my moral reasoning than I'd like to admit. For this example, just because you want to use the parts/bike and no one else is, it doesn't mean it's right for you to take it.