THE LATEST visible result of the Sadat-Begin dialogue - President Sadat's call for a pre-Geneva preparatory conference in Cairo as early as next Saturday - further attests the seriousness of their joint effort to move toward a Middle East settlement. They are determined to keep up the momentum of the breathtaking Sadat visit to Jerusalem a week ago.

They are doing so, morever, within the context of a quest for a general settlement, not one limited to Israel and Egypt. Mr. Sadat does not want to lend substance to the charge of some fellow Arabs that he is betraying the Arab cause. They may yet leave him no choice but to go for a separate deal. If so, Mr. Sadat would no doubt present it, honestly, as the initial installment on a general settlement. No reasonable person could fault him for it.

Who's reasonable? A good test is to see who attends the Cairo conference. Syria and the Palestine Liberation Organization have said they won't go - a foolish decision that puts them openly in the position of wreckers. The PLO risks leaving open the Palestinian seat at Cairo, and perhaps later at Geneva, to representatives of West Bank and Gaza Palestinians. Mr. Sadat, with evident Israeli encouagement, has publicly reached out to them. Jordan lacks real freedom of movement in intra-Arab disputes. But it is obvious that the Sadat-Begin way matches King Hussein's long readiness to resume a West Bank role.

The Soviet Union is chewing its Cairo invitation over. Its choice is to board the Geneva train in Cairo and use its influence to bring about the compromise settlement it professes to seek or to succumb to the temptation to join the Arab wreckers. If it goes with the wreckers, it will show itself conspicuously unworthy of the tentative confidence the Carter administration expressed by working out the joint Soviet-American Mideast statement of Oct. 1. This would have implications extending beyond the Mideast. The administration ought to say so.

The United States evidently leans toward Cairo but hopes that all the Arab invitees, and the Russians, will be there, too. The administration is certainly right to canvass all the possibilites. Yet its hesitation nourishes suspicions that it is less than enthusiastic about a Mideast process, no matter how promising, that it did not initiate and that it does not fully control. There is a certain smallness to the administration's response to this latest sequence. The President himself, we think, ought to personally address it.

It will be interesting to see if the United Nations' Kurt Waldheim, a firm supporter of the General Assombly's right to make one-sided propaganda against Israel, will show up in Cairo in order to put the United Nations on the side of a helpful diplomatic initiative. The General Assembly, you may have noticed, taking up an anti-Israel resolution put in the hopper some time ago, did its grubby little act again on Saturday. We missed Mr. Waldheim's statement of regret.

The Israelis accepted the Sadat invitation almost before it was out of his mouth. It had obviously been worked out in Jerusalem. No less important, however, is the developing evidence that Israel accepts the obligation the Sadat trip put on it to review its own past policies. A few days ago Foreign Minister Moshe Dayan, no dove, appealed to Israelis to review their thinking in the territory necessary to hold in a peace. A spokesman Saturday said merely that Mr. Sadat's demands for full territorial return and Palestinian self-determination "contradict" the Israeli position; he did not say those demands cannot be met. As Mr.Begin himself said, everything is negotiable. Otherwise, it would hardly have been worth Mr. Sadat's while to come to Jerusalem.

Even while Mr. Sadat was there, some observers - and some highly placed American officials - insisted on suggesting that the Israelis had to "give" Egypt hard coin, on the spot, so that he would not go home "empty-handed." As it turned out, Mr. Sadat himself spurned this simplistic logic. What he wanted, the Egyptian said, was for Israel to rethink its whole position - something he realized could not be done overnight. The evidence that Israle is engaged in a fundamental policy review, on the official level and on the public level, too - is the best reason for hoping the Sadat-Begin initiative will continue moving along.