If you mean when JMB members will be able to accrue miles that count for oneworld awards, that's already true. When JAL joins oneworld current JMB mileage balances could be used to redeem on oneworld airlines. JMB elites will have certain privileges when flying oneworld carriers, and there will be significant reciprocity. (First Class lounge access will be interesting, though. JAL's own top elites don't get that. It's for paid F.)

If you mean when JMB members can earn JMB miles by flying oneworld airlines, that will occur when JAL joins oneworld. The rumored date is April 1, 2007, but that's only because Aer Lingus departs oneworld at that point (and thus removes any need to link JAL and Aer Lingus systems, which is expensive to do if it only lasts a short while). It could be later than April 1.

Now, it's up to JAL whether they want to award elite status miles (called FLY ON points) to JMB members when they fly oneworld airlines, and at what rate. None of that has been announced. I suspect it won't be any more than 1 FLY ON point per mile flown (with some uplift for paid J or F flying), but it could be even less than that. It's possible there will also be a minimum JAL FLY ON requirement for elite status. All speculation, though.

It is possible for JMB members to earn miles (but not FLY ON points) when flying American Airlines, for example. There are other airline partners. There are also codeshares, and the codeshares do earn FLY ON points.