IT IS POSSIBLE to say, very cautiously, that things are looking up in Lebanon. Not only is the cease-fire proclaimed earlier this month between Christian militias and Syria's "peacekeeping" forces holding, but it was also substantially strengthened over the weekend when Saudi troops replaced Syrians at key locations in Beirut. The Christians, who probably provoked a good part of the Syrian onslaught on them, detest the Syrians but find Saudis acceptable. In return for the switch of peacekeeper, they pledged to cool down. At the same time, Lebanese army units are starting to take up peacekeeping duty in Beirut. Since the ultimate point is to phase out foreign peacekeepers, who have been on hand since the 1975-76 phase of Lebanon's civil war, the entry of Lebanese soldiers has a special appeal.

Underneath the surface, moreover, there are signs of stirring. The Syrians may be losing some of their taste for intervention in Lebanon. For trying to keep the peace in their fashion, they have been roundly condemned for savagely attacking first Palestinians and then Christians. They have been unable either to fold Lebanon into a Syrian design or to steer Lebanon toward an orderly future of its own. With Egypt and Israel about to make peace, Damascus may feel dangerously exposed to maneuvers by the Israelis and their Lebanese Christian clients. Little wonder that Syria seems ready to step back a pace in Beirut and let other Arabs assume some part of a thankless peacekeeping mission. The Israelis, helping in their fashion, have stepped back a pace, too.

The truly interesting aspect, however, is the role of the great powers. Lebanon is perhaps the only trouble spot going where Moscow and Washington are, if no cooperating, pursuing somewhat parallel policies aimed at stability. The theory that the Russians were fanning the flames in Beirut to undermine Camp David faded when they backed the cease-fire. Frozen out of the Arab-Israeli action, Moscow apparently wants in Lebanon not a wider war but an opportunity to act on a Mideast diplomatic stage.

Jimmy Carter is taking a personal interest in American efforts to calm Lebanon and explore the shape of an internal solution. Vice President Mondale, questioned recently, pointedly declined to put blame on Syria, whose cooperation will be crucial to any easing of the poisonous religious, class and personal disputes that make up Lebanon's tragedy. France is also constructively engaged in the quest for stability.

Lebanon has given "cease-fire" a bad name, and this one may hold up no better than its countless predecessors. But somehow we have a sense that the stage - the U.S.-Soviet-French international stage - is set differently this time around, and that the behavior of thos directly engaged (Christians and Syrians and to some extent Israelis and Palestinians) may as a consequence be significantly different, too.