I've now gone 9 days since I've switched to Diet Coke, and I've not cheated yet. I may have cut down my caffeine intake slightly, as well (it's possible, but it wouldn't be by much). My Coca-cola habit was pretty bad, I'd say generally speaking I have cut about 900 calories a day from my diet by switching from Coca-cola classic. That's over 2 pounds fewer calories over the past 9 days (though I don't think I've actually lost 2 pounds).
I have also changed my diet a little (it's a slight change, not a "diet"). I am doing the Subway for lunch thing - I walk through the downtown tunnels to the subway by Beck's Prime (best burgers in Downtown, but definitely expensive) and I get a six inch sub from their "Low-fat" menu. I do still get cheese on the sub (so it's not the "less than 6 grams of fat"), but not the mayonnaise, oil, or vinegar. I also am only eating a 6 inch instead of the foot longs that I used to have.
Dinner I do the same. For Breakfast, I'm actually eating a little now, instead of purely a liquid breakfast. I don't know how much help that is, but it's probably better than nothing.
At any rate, I'd venture to say I've managed to cut close to 3 pounds' worth of calories from my diet by making these little changes. If that's the case, then maybe I'll be a little trimmer when I fly out in 26 days.