Learn English – What’s the difference between “Thanks anyway” and “Thanks though”

@RegDwight is correct in every particular. Till is indeed older and the two can be used pretty much interchangeably.

There is really only one usage I can think of where you would not use till: in setting up a negative conditional.

Until my landlord fixes the plumbing, I am not going to pay the rent.

I am not going to pay the rent until my landlord fixes the plumbing.

Here it would sound strange to use till, and I'm not really sure why. Perhaps it is the negative.

One more thing. Sometimes you will see people spell till as a contraction of until: that is, as 'til. This is simply wrong, and not seen outside of old poetry and modern greeting cards aiming for a "poetic" tone. Anywhere you see 'til being used, till would be the correct word.

Ignoring the grammarian 'it is wrong' response, the 'standard' (for want of a better term) answer is that it is a quasi-passive. Fowler, for example, explains it as such in his Pocket Modern English Usage. The basic idea is that sentences such as "someone broke the car" and "the car needs fixing" are passive-like in function though not form as the actor is external and/or unknown.

Maybe more relevantly, the same idea allows the passive voice to have a continuum of function for the past participle from adjective-like to verb-like. In this, sentences like "I'm sat/I'm stood" are more adjective-like in function while "I'm sitting/I'm standing" are obviously more verb-like.

In short, "I'm sat here" is similar to "I'm big" or "I'm tall" - you are describing yourself more than saying what activity you are engaged in. In contrast "I'm sitting here" is saying what you are doing.

In the spirit of being fair, I am not 100% convinced by this explanation but neither can I think of a better one.