Ask HN: Internships - paid or not paid?

I recently saw the "Who's Hiring" thread for January, and I see a lot of companies I am very interested in! Currently I am seeking an internship in California for the summer, and I would like to notify the startups of my interest.

Here is the pressing question I have: are most of the internships paid? Most of the time, information of pay and/or stipends is omitted. I understand why, but it is still important information I need to have before going through the whole interview process.

From a startup perspective, I can see two reasons why these startups would want to hire interns. 1) Finding engineers is difficult and the startups will take anyone if only for three months. 2) Free labor, which is especially helpful in an work environment where money could be scarce. Hopefully for the starups I am interested in, it's the first reason.

I currently have a comfortable, well paid job, so I don't want to leave it with the understanding that I could be applying for an unpaid internship.

Also, since I need to ask about pay or stipends, what is the best time and manner to do so (in the interview, by email, etc)?