You should now use the AllowPageShuffle property. This change is consistent with the UI change. In CS2, the Page's palette offered "Keep Spread Together" and so the property was Spread.KeepTogether.
In CS3, the comparable command in the Pages panel is: "Allow Selected Spread to Shuffle" and so the object model has been changed.
Data Type: boolean
If true, guarantees that all new spreads added to the document contain a maximum of two pages. If false, allows pages to be added or moved into existing spreads. For override information, see preserve layout when shuffling.
I wonder where we're supposed to find "preserve layout when shuffling"? And clearly, this is a description of the document preference, not the spread option.