You might want to get Toddgannit (sp?) involved in this thread. He does a lot of work with limited equipment and bodyweight + weight type workouts.
I think as long as your dumbbells are heavy enough such that they provide adequate resistance and you do not outgrow them in the next year you should be able to make steady gains with a good program. Also, keep in mind stagnation occurs following 3-4 weeks of doing the exact same thing, so you should think about ways to progress in intensity, volume, or exercise.
Jason Cholewa, Ph.D., CSCS