I've just double checked verbix and (of course) there is no 'was' version of the subjunctive, but I'm starting to suspect that even in the indicative, there is no possibility of using 'was' in the past tense with an 'if' statement. Perhaps a language expert can confirm?

[quote="goofy"]My idiolect doesn't make any difference in meaning between

If I was the Prime Minister, I would change the law.
If I were the Prime Minister, I would change the law.[/quote]

But wouldn't the first one require the second clause to also be in the past tense i.e. "If I was the Prime Minister, I would have changed the law."

Whilst language does indeed change over time, and I'm a firm believer in descriptivism (for native speakers), I don't agree that (all) grammatical concepts should be eliminated just because of a few uneducated people because without these concepts, we will end up speaking pigeon English; unable to communicate a wide enough range of ideas. The only reason people confuse 'was' and 'were' in that construction is because of a lack of education and understanding of what the purpose of the subjunctive mood actually is. If the subjunctive disappeared altogether, it would make for extremely confusing communication at times.

I would also like to point out to the people screaming for a 'no rules' and 'anything goes' version of English: I could understand wanting 'was' to be equally acceptable [i]if the subjunctive were actually no longer being used[/i] but that isn't the case. Just because you don't realise you are using the subjunctive, doesn't mean that it isn't used. We use it just as often as other languages do but as we recycle words (a lot), it isn't always obvious. Until the subjunctive actually stops being used, using 'was' instead of 'were' remains an uneducated mistake as opposed to a sign of language progressing(/devolving).