In my experience, for the meaning of "to be armed," the expression is only "to pack (or, to be packing) heat," without the word "some." When that word is added, it changes the meaning, so we would need context to know the meaning (as requested back in 2008).
Ex.
1. The 1978 Chevy Nova police car packed some heat under its hood. This car had everything you might think a police officer would want.
2. [In the Thai dish,] the veggies were fresh and the chicken tender, and it packed some heat.
In #1 it means that the engine was powerful. #2 means that the food was spicy (hot).