Depends. I tend to define a swing state as a place with regular changes in the state legislature, regular shifts in Senate and Presidential elections, and registration more or less evenly divided. Missouri and Florida are currently "swing states" by my definition. My home state of New York, which has a long history of alternating between RINO and Dem-Socialist governors, cannot be considered a swing state because 1. Margins of victory for Dem politicians are usually considerable. 2. Dems outnumber Republicans by a wide margin and 3. the only Pubbies who get elected in New York are those who govern like Democrats.
New York State Politics in a Nutshell: Republicans = Democrats, Democrats = Socialists.
I'd agree that Missouri and Florida are swing states. I wouldn't consider New York, Texas, or California to be swing states.
As a rule of thumb, if both Presidential candidates don't campaign and run ads in a state, they've conceded it as a non-swing state. Token appearances don't count.
I suspect the GOP has written off California for the indefinite future. President Bush certainly didn't waste time and money there in 2004 like he did in 2000. And unless and until the hispanic vote in California for the GOP approaches 50%, the demographics only look worse in the future.