It goes back to the specialist vs. generalist question. An MBA experience will help you become more of a generalist, you will interact with people who are different from you and solve problems differently. It will also open up career options you may not have today, hedge funds, investment banks, consulting - some of which are intellectually interesting and pay well. Even if you decide to do a startup later, you will have friends working at VCs & operating companies that you can get help from.
If on the other hand you have decided that you want to live the rest of your life as a hacker, maybe it is not such a good idea. In any case, don't consider schools beyond the top 5 (Stanford, Harvard, Wharton, Chicago and maybe MIT).
The thing is you don't need an MBA to be more of a generalist. You can read all the class material at a fraction of the price. Not to mention in SV you have a lot of other venues for networking.
I talked to a graduate of Stanford's program about his opinion on it. He told me the biggest value the program gives you are new contacts (He didn't really think the class content was anything special), but since he didn't want to go to Wall St/ or to Big Corporation X (where the cred helps at times) he felt that he should have just skipped it and instead just dived into a startup (using his MBA school money for the venture instead).
Good MBA programs typically cost from $30000-$50000 per year and it doesn't even include opportunity costs.
If on the other hand you have decided that you want to live the rest of your life as a hacker, maybe it is not such a good idea. In any case, don't consider schools beyond the top 5 (Stanford, Harvard, Wharton, Chicago and maybe MIT).