Unless I am mistaken I am the first to create a utility that does exactly that : taking a European PAL TV show recorded with (Vista) Mediacenter, ripping the subtitles out of them, and putting them into an SRT file.
Now, I realise this is not ideal, as Teletext subtitles have much detail that is being lost that way :

* screen position
* colours (sometimes 3 different ones are used on the same page, and there might be several per line).

So now I am wondering how I could implement that.
Screen position is maybe not that important, but colours are, as often another colour is chosen per actor.

Subtitles for one screen might look like this :

Yes ? Yes !
Radio plays in the background

So my question is : can I do that with SSA ?
Should I use SAMI ?
Or still something else ?

My first beta (only useful for European and other users in PAL countries) is at :

http://www.extrabuttons.net