Intel wouldn't have come out with dual cores as quickly, but it would have happened sooner or later. The main reason (aside from competition from AMD) why they went down that route was because their prototype Tejas chips needed something ungodly like 200-250W to clock at speeds even comparable to Prescott, and were apparently even blowing out the VRM circuitry on test motherboards. Out of the options on the table, switching to Merom/Conroe would have been the least expensive option for Intel regardless.
In any case, without competition from AMD, chances are that the Core 2 Quad would have never existed. The main reason why Intel produced that so quickly was pretty obviously to get revenge on AMD for the three year ass-kicking it had sustained, so without the (theoretical) competition from Phenom, odds are they'd have waited for Nehalem to show up before going quad-core.