Yasser Arafat's visit to Egypt has so far sown new disarray on two Mideast fronts: the Arab-Israeli negotiating divide and battered Lebanon. The ultimate result of Mr. Arafat's reconciliation meeting with Egyptian President Hosni Mubarak will depend partly on the next moves of the PLO chief and other Arab leaders. Yet by Monday, there was considerable skepticism in the Mideast that - whatever the ultimate political ripples of the Cairo talks - peace will draw closer in either area.
In Washington over the holiday weekend, President Reagan spoke more optimistically. He ventured that ''enough progress has been made'' in piecing Lebanon back together to convert Arafat's encounter with Mr. Mubarak into a new impetus for wider Arab-Israeli negotiation.
But as if in rebuttal, the Lebanese capital of Beirut erupted in its fiercest fighting for several days as the Army vied with Shiite Muslim forces for positions vacated by the French peacekeeping troops. Another opposition militia, headed by Druze chief Walid Jumblatt and backed by Syria, was also reportedly shelling Army positions and other targets in and around Beirut.
On the Arab-Israeli front, Arafat's move - apparently made without consulting close allies within the Palestine Liberation Organization - has drawn a storm of criticism from fellow Palestinians, and only hedging comment from the Arab oil world's chief power broker, Saudi Arabia.
The early lesson of all this seemed that, notwithstanding Mr. Reagan's remarks, the problems of Lebanon and Arab-Israeli negotiation would be separated only with great difficulty.
Arafat's trip to Cairo - only days after he and guerrilla loyalists were evacuated from siege in north Lebanon at the hands of rival, Syrian-backed Palestinians - was interpreted by friend and foe as an implicit threat to enter a go-it-alone peace process in defiance of Syria.
The latest fighting in Lebanon - where Syria keeps some 40,000 troops and backs antigovernment forces - seemed a prompt public reminder that Damascus has ample ways of responding to any such move.
One Palestinian political analyst and longtime Arafat supporter saw the Lebanon crisis as a main explanation for the Saudis' hedging comment on the PLO-Egypt talks.
''The Saudis surely welcome the idea of a more moderate Arab consensus built around an Arafat-Egypt reconciliation. But the Saudis are also at the most delicate stage of their attempt to mediate some sort of peace in Lebanon in conjunction with the Syrians,'' he said.
The Saudis are said to have given their blessing to the gradual, limited reintegration of Egypt into the moderate Arab world in the past few months - a process whose latest reflection came Sunday, when Jordan formally ended a trade ban imposed with other Arab states after the late Anwar Sadat's peace with Israel.
In Lebanon itself the Arafat-Mubarak talks and Mr. Reagan's subsequent comment can have caused only foreboding within the hard-pressed government of President Amin Gemayel.
For some time, sources close to the government have expressed fears Reagan would extricate his vulnerable United States Marine contingent and leave Mr. Gemayel with the daunting task of sorting out Lebanon's various rivalries. This kind of talk has persisted in Beirut despite public US efforts to dampen it.
Whether by design or accident - and some Palestinian analysts suspect the latter - the Cairo visit seems to have moved the PLO closer to a formal split between pro-negotiation and ''rejectionist'' components, something Arafat has for years acrobatically sought to avoid.
Opposition to the Cairo trip from Palestinian leaders George Habash and Nayef Hawatmeh - both of whom had refused to go along with the Syrian-backed mutiny against Arafat in the hope Arafat would afterward shelve any moves toward US-mediated negotiation - was a foregone conclusion.
But Arafat's decision to make the visit before a formal meeting of even his own Al-Fatah guerrilla faction has drawn criticism from that quarter as well. This raises at least the possibility he could find himself at the head of a smaller and far less credible voice of Palestinian political sentiment, but one more united than his previous coalition of groups.
This, in the US view, might not necessarily be a bad thing. The principal US hope is that a reoriented Arafat may provide enough political cover for Jordan's King Hussein to enter talks with Israel on Mideast peace.
Various key moderate Palestinians in the Israeli-occupied West Bank and Gaza Strip harbor similar wishes. But they also hope and assume Arafat will weather the current inter-Palestinian storm better than naysayers predict.
The first sign of the ultimate effect within the PLO of Arafat's Cairo visit - or of whether he may now do some backtracking in the interests of shoring up his position - could come in North Yemen, where he was said Monday to have summoned the PLO's military council.
Yet even some moderate West Bankers find it hard to work up much optimism. As one remarked, ''No matter what Arafat does or does not do, the Israelis do not want a settlement even along the lines of the most minimal, moderate Arab demands for Palestinian self-government. . . . If the Americans, a superpower, haven't delivered such a settlement in the five years since Camp David, what can Arafat, or King Hussein, much less we unarmed West Bankers do to achieve this?''