“my tire flattened”

Never. But I might say "My tire went flat."

I must say some of your other examples are not convincing. "The weather changed" doesn't seem nearly as transitional to me as "The storm weakened."

mara Oct-7-05 6:31PM

"Flatten" usually means "make flat" or "knock down".
http://www.answers.com/flatten
It rarely means "become flat", though I could (barely) imagine describing a musical note as flattening if it became lower.

But I'd never use "flatten" to mean "go flat". A tyre goes flat, but it doesn't flatten.

"I liked him within a minute" seems wrong, but for a different reason -- liking isn't a transition. All your other examples seem both okay and unrelated to the flattening question (except that "ripping his script up" would be better as "ripping up his script").

then, MPT, could a nail flatten my tire?

Now, after the simplest of research, I can say that it is perfectly ok to say "...my tire flattened...". The transitive verb to flatten means to make flat. The intransitive verb to flatten means to become flat. Simply check any dictionary to confirm this. There are a great many verbs that work this way:
I ripen a banana by putting it in a paper bag, but a banana ripens in the bag.
I open or close a door but a door also opens or closes. I melted an ice cube. The ice cube melted.

As an english speaker (American), the sentance makes complete sense to me, and doesn't even really seem akward. It may not be what we would normally say, but I think it's just fine.

I'm an American (currently living in Las Vegas) and have heard several different phrases for explaining the condition of a tire (not spelled "tyre" in the US):

I got a flat tire.

One of my tires went flat (the "One of" at the beginning sounds better unless you only have one tire to begin with as with a unicycle).

One of my tires blew out (if the tire exploded or was shredded).

Use of "flattened" is also correct. For example, "My tire flattened as a result of the many puncture wounds inflicted upon it by Bruno," is grammatically correct and doesn't sound awkward to me in the slightest. It might not be used in everyday conversation, but would sound great in a novel or other piece of writing.

The other use of flattened (my heavy boot flattened the squirrel) can describe what happens to a tire as well. "The 20 ton steam roller flattened my tires (and the rest of my car as well)."

No actual tires were harmed in the writing of this response (just a squirrel).

I've heard "flatten out" or "flattened out" a lot in colloquial usage, but that doesn't make it proper. It's not a rule that every transitive verb describing changing the state of something can be intransitive as well, is it?
For example, you wouldn't usually think of "to cut" meaning to be cut. But it can be said that paper cuts well or cuts roughly. However, is that proper?
A better example: "heightened". I've never heard that used as an intransitive verb, have you?

More examples:
folded (which is proper)
thinned
rounded (out)
filled (out or up)
fattened (up)

I wrote the sentence under discussion, and just discovered this discussion.

The American Heritage Dictionary, 4th edition, includes a definition for "flatten" as an intransitive verb, meaning "to become flat or flatter"; OED, 2d edition's entry includes "to become flat, or more flat; to lose convexity or protuberance; to grow broad at the expense of thickness."

I feel I must side with myself on this one. It's acceptable usage.

Could you ask your garage:
Is my car tyred, meaning : are the New tyres fittid?