Re: I refer to a non-specified carbonated beverage as a:

[CodeJunkie]

I'm from the UK and I voted for fizzy drink, that's only because using a process of elimination it's the only one I would vagely consider using.

The way I see it:

Pop - Something you might say if you were about 5
Soda - Something you would say if you were American
Carbonated Beverage - Something you would say if you were a maths geek or a scientist or something.
Tonic - Something you mix with Vodka
Coke - That's a bit specific, you can hardly call a Sprite a coke now can u! That's just stupid!
Soft Drink - Well yes that is ok, although it doesn't specify if the drink is fizzy or not. You can have still soft drinks.

Therefore I am only left with fizzy drink (really only because it clearly describes what we are talking about)!:-)

[Anonymous Monk]

For me it's just a "drink" -- other drinks must be specified, carbonated is default.

So I picked "other", which is honest. Often I wave my empty glass or bottle and say "I'll have an Other."...

[snax]

Coke - That's a bit specific, you can hardly call a Sprite a coke now can u! That's just stupid!

Go to Atlanta and ask for a Coke at a diner. I'll give 3-1 odds the waitress says something like "OK, sugar, what kind of coke would you like?" -- they really do use it as a generic term (think xerox or kleenex) in that part of the country. It's a bit bizarre if you aren't used to it.