From Haaretz:

A government source in Jerusalem said the IDF received instructions to proceed with the operation after officials viewed an updated version of the draft of the cease-fire resolution Friday, which included several points which Lebanon backtracked from one day after agreeing to them as spelled out in Thursday’s draft.
A government source in Jerusalem said that Israel agreed to a 48-hour lull in order to exhaust diplomatic efforts, it became clear that progress towards a cease-fire was not as significant as thought Thursday.
The draft of the text which arrived from Lebanon on Friday is a significantly altered version of Thursday’s draft. According to the modified document, Lebanon is opposed to a beefed-up contingent of UNIFIL troops and granting UNIFIL the authority to enforce the cease-fire, which is more problematic for Israel than the clause which refers to the Shaba Farms as well as the lack of border supervision over weapons smuggling.
Under these circumstances, the source said, Olmert decided to issue the order for the ground assault while continuing diplomatic negotiations in parallel.
Olmert’s spokesman, Asaf Shariv, told The Associated Press that the expanded incursion had already begun. A cease-fire deal being worked out by the Security Council fails to meet Israel’s basic requirements, such as stationing robust international combat troops in southern Lebanon once Israel withdraws, Shariv said.
“Yesterday we were very optimistic, but they (the Security Council) took the wrong turn,” Shariv said.