I have a BA in English, and too much programming experience. I'm working with three local dev firms and have freelance projects to keep me busy.
Do firms really care if I have a degree, or do they just care about the language I know and if I can deliver?
Usually depends on the size of the firm and the position. Smaller groups will often be a bit more flexible as long as you can demonstrate real skill. It you don't have experience and you lack a BS that's trouble. Larger companies with an actual HR department will often require a BS, though you can still submit your resume and sometimes get an interview. The most important thing is your ability to actually do the work so real world experience goes a lot further than a degree. Education is important either way - whether obtained from a formal institution or your own research.
They didn't care back in the DotCom days. They just wanted you to have a skill. But nowadays they do care. Back then I would say that only 1/3 of the jobs required a degree. Now it's more like 2/3.
It's not hard to verify this ratio. Just follow the job boards.
NOTE: Your BA in English counts as a degree.
You'll run into 3 kinds of companies:
There are plenty of all three out there. Decide what you really want to do and go that way.
I think in the first few years of software development, a degree is looked at with pretty high importance. I think employers want to know that you are willing to work hard, stick to a program, and acheive goals.
After about 5 years or so, I don't think employers view the education part of your resume as highly, often using it just as a simple filter to slim down the number of applicants they have to look at. Work experience becomes very important from then on.
Graduate degrees are helpful if there is a specific field you would like to specialize in, or if you would like to as some point become a professor or teacher of programming.
A degree is usually important/helps when you don't really have any demonstratable ability under your belt. Given that you say you have a fair amount of programming experience and have worked commercially as a developer i don't see this being a real problem.
A lot of the time the just like to see a degree as it proves that you can commit to something for ~4 years and have the ability to learn/be taught.
I care about the education that you (should have) received as part of getting a CS degree. Learning fundamentals of the field can really help you become a better programmer over time, as your experience catches up with your theoretical knowledge. If you have (can get) the same understanding without the degree, it wouldn't count any less to me. You probably will have some doubt to overcome, though. At least, with a CS degree I know that you have been exposed to the fundamental topics and should, therefore, be able to build upon them.
I am guessing that you're asking about an undergraduate degree rather than a graduate (MS/PhD) degree. Is it necessary? No. But in a competitive market, it's important.
Larger companies don't just want someone who knows how to program. They need to feel comfortable that the person got his "core CS" such as algorithms and data structures and complexity. Never mind that you can easily learn all the important stuff from books like CLR, most places want it on paper.
The bigger is that there is also a prejudice against degrees that are not CS-centric or closely related. Not everywhere, but it does exist. A major in the humanities is (unfairly) a strike against you in some companies and HR department, possibly because many HR personnel also have a degree in the humanities. One way to "cover the shame" is to get a graduate degree in CS since many places will look only at your last degree.
Do firms really care if I have a degree, or do they just care about the language I know and if I can deliver?
Firms want to know if you can deliver, and they want to see evidence that will convince them of this. If you've got good experience, that will count. One of the big values of a CS degree is in getting a foot in the door in industry, by demonstrating that you have some (small) amount of relevant experience.
That said, you will run into companies that won't hire people without a college degree (which you have---English counts). I wouldn't be surprised if there are individual managers or HR departments who will refuse to hire people without a CS degree. But that kind of narrow thinking is probably a sign of a place you don't want to work in the first place...
One of the best programmers I worked with on a recent project was, in fact, an undergrad English major. He didn't have any graduate degree that I was aware of. He was recently laid off and didn't have much trouble finding an interesting new position, despite the economy.
I've also worked with excellent programmers with degrees in physics, mathematics, political science, and philosophy.