What do
you want out of this?
Unless you can answer this question, any advice is essentially pointless. In my experience, you can imagine up all the scenarios in the world (I know I have many times), but reality depends on so many factors outside your control that things are likely to go in a direction you do not expect anyway.
Regardless of whether you want be acquired or not, you should take that meeting. You have nothing to lose, and everything to gain: Even if nothing comes of it, it will be an opportunity to gather valuable information, create useful connections that could be useful later on, or lead to other business opportunities that aren't even on your radar right now.
Most importantly, you need to look at this and every other meeting you take as the beginning of a negotiation—even if you never discuss an actual business opportunity. And the trick with being a good negotiator is not necessarily to win, because nobody wins all the time. Instead, the trick is to stay in control of the process. Talk to these folks, and see what they want. A company that is serious about acquiring you will want to get to know you first, understand your strengths and weaknesses, and so on. Your job is to steer the discussion in a direction that's worthwhile to you and never, ever, ever, ever (ever!) let it take more of your time than it deserves.
So many of the other folks who have commented seem to worry about this being a waste of time, or the other company attempting to steal your ideas and businesses through the due diligence process. But how else are you ever going to get a deal done if you don't interact? In my experience, due diligence is the last step of an M&A exercise: after the parties have agreed on all the terms, its role is to simply make sure that neither party is trying to pull one over the other. And it's an expensive process, so no-one really wants to do it until they're sure they want to go ahead with the deal.
So, nobody is going to start a serious negotiation by asking to look at your financials or read through your source code (and, if they do, you should probably walk away) until they are willing to put a commitment of their own on the table. In the meantime, just let the discussion take its course and push back, firmly but politely, every time you feel that the process asks more of your involvement than you feel is warranted at that particular time. Again, any serious businessperson won't mind—heck, they may even try to measure up your worth by seeing how far you're willing to be pushed before you start pushing back.
Oh, and you should run—_run_—and find a good advisor. This is the kind of discussion you want to sound off with someone who can help you examine your specific situation… which is hard to do in a public forum like HN :-)