By Jonah Fisher
BBC News, Johannesburg
Morgan Tsvangirai says the suggested deal is one compromise too much
In the run-up to Sunday's summit, hosts South Africa had promised to talk tough with the Zimbabwean parties and "force" an agreement.
Many observers interpreted that as a sign that the region's most powerful country was at last going to pressure President Robert Mugabe into making concessions to the opposition.
In fact, as the dust settles on the talks it is clear that the exact opposite was the case.
Despite winning the only contested election in Zimbabwe this year, opposition leader Morgan Tsvangirai was yet again being urged to compromise by southern Africa leaders.
Tomaz Salomao, the head of the Southern African Development Community (SADC), which organised the summit, announced that an "above normal" situation meant that the home affairs ministry should be "co-managed" by two ministers, one from President Mugabe's Zanu-PF and one from Mr Tsvangirai's Movement for Democratic Change (MDC).
The joint share of home affairs would go alongside most of Zimbabwe's key ministries in the Zanu-PF portfolio.
Effectively SADC was instructing the MDC to accept Mr Mugabe's definition of power-sharing - that they should take a junior role in his government.
"These regional organs are state to state," David Monyae, a South African analyst, told the BBC. "The idea of opposition groups coming in and getting heard is not something they are comfortable with."
So as midnight passed in the Sandton Convention Centre, Mr Tsvangirai was cast as the obstacle to a settlement. At his press conference he came out fighting.
"It is about equitable power-sharing," he said. "It is about giving the responsibility to the party that won an election and has compromised its position to share a government with a party that lost."
Will President Mugabe go ahead and form a government?
The MDC made it clear that they would not take part in the government on those terms.
In demanding that Zimbabwe's leaders accept their deal, SADC tried to flex its muscles - and the MDC's rejection has left it looking weak.
"The problem with SADC is that went into the summit this weekend without any clear alternatives," Judy Smith-Hohn, a senior researcher at the Institute for Security Studies, said.
"Those people who were at the summit didn't have time to strategise and that's why they came up with this result."
For a meeting that was billed as "make or break" for the Zimbabwean deal, the turnout was low. Just five of southern Africa's 15 heads of state attended.
Significantly, the president of Botswana, Ian Seretse Khama, was absent. Having criticised President Mugabe and called for fresh elections, it is hard to see how he would have accepted the summit's final verdict.
Mr Mugabe can now claim with some justification that SADC has mandated him to form a government immediately. Over the next few days he may well do so.
This leaves the opposition with an impossible choice. Whether to struggle for power from outside Zimbabwe's government or belatedly battle for a share within.