From Eagle Brands:
For an easy caramel topping or dip, simply heat Eagle Brand Sweetened Condensed Milk using the following directions. For safety reasons, do not heat in the can (an old cooking method). Instead, use one of the following methods.
: Pour 1 can Eagle Brand Sweetened Condensed Milk into a 9-inch pie plate. Cover with aluminum foil; place in larger shallow pan. Fill larger pan with hot water. Bake at 425° for 1 1/2 hours or until thick and caramel-colored.
: Pour 1 can Eagle Brand Sweetened Condensed Milk into top of double boiler; place over boiling water. Over low heat, simmer 1 to 1 1/2 hours or until thick and caramel-colored. Beat until smooth.
: Pour 1 can Eagle Brand Sweetened Condensed Milk into 2-quart glass measuring cup. Cook on 50% power (medium) 4 minutes, stirring briskly every 2 minutes until smooth. Cook on 30% power (medium-low) 20 to 26 minutes or until very thick and caramel-colored, stirring briskly every 4 minutes during the first 16 minutes and every 2 minutes for the last 4 to 10 minutes. Make sure to use potholders when removing the glass measuring cup from the microwave, as it will be hot.
: DO NOT HEAT IN CAN.
I've never used any of these methods but it's on my list of things to try. =)