No, I didn't drop anything or knock something over. However, when the bottle suddenly deformed the extra pressure caused a small jet of Dr Pepper to overshoot the glass and land on the laptop keyboard.
Only a couple of drops made it through the keyboard into the laptop, and they landed harmlessly on metal surfaces. I have cleaned the keys which were sticking, and all the keys still work, as does the TrackPoint pointing device.
So why is it that my right-hand mouse button no longer works, when the spillage didn't actually go near the mouse buttons? (If I had wanted a computer with only one mouse button I would have bought a Mac.) I can take the mouse buttons off to expose the little membrane circles underneath; they are clean and look identical, but only one of them works.
The keyboard itself (including the mouse buttons) seems to be a sandwich of two pieces of plastic with a membrane in between (and I should think anyone who has ever had a ZX Spectrum knows what a keyboard membrane looks like). The bottom piece is just a base and the top piece is what holds all the keys on. Unfortunately, separating these pieces would involve breaking the plastic rivets holding them together, which means that putting them together again afterwards might be tricky. The membrane itself (or what I can see of it underneath the keys) looks to be of the type that should be resistant to small amounts of liquid spillage, so the loss of my mouse button is a bit perplexing. (I suppose it could be a coincidence.)
The IBM parts shop website does not acknowledge the existence of my keyboard (well, the laptop is over six years old now) but it does recognise the adjacent part number, which is the equivalent keyboard in US layout. Unfortunately it quotes the part at 96 pounds plus VAT, which is probably at least half the current value of the laptop!
Oh well. A laptop with a PS/2 mouse plugged in is still a lot more portable than a desktop, although it could be rather harder to use in a confined space (such as the tiny table it normally rests on at home).