A lot of people here are saying stuff about PHP, .NET, java, whatever.
But I'm going to make a new argument:
For the web, the server side is fast becoming irrelevant, especially when you consider your criteria (the interactivity of SO.COM). You want to learn 4 core technologies. And if you think you know them or can figure them out without study, you're probably wrong. The standards have evolved for too long, and it's way too easy to get a clouded picture of "how things should be done" if you just try to learn by rote.
1) Modern, STANDARD (x)HTML. Know what's in and what's NOT in HTML/xHTML 4. Think you know it? Do you know the difference between a block level element and an inline element? Do you know which elements are allowed in which other elements?
2) CSS 2.1 or greater. I recommend O'Reilly's "CSS: The Definitive Guide". Know CSS selectors inside and out. You'll need them for Technology #4. You'll be stunned at what can be accomplished in CSS, which you thought you could only do with tons of tags in HTML.
3) Javascript. No, I'm not kidding. It's no longer a joke language that doesn't work. It works. It's what's running the most responsive, most interactive, and interesting sites around (Google Maps, SO.COM).
4) JQuery. It's so glorious I find myself not able to even speak of it. Maybe the commenters will be less overcome with it's glorious aura and will be able to breathe its name.
Since all of the really impressive stuff, IMO, is on the client side nowadays, the server-side language doesn't mean as much. I love .NET's LINQ syntax. I doubted it at first, but I am drinking that Kool-aid by the gallon these days. But if you choose PHP or RoR or Java or whatever, fine. But that should be a secondary decision. Learn the 4 technologies above first -- you'll need them no matter what you're doing on the server.