There is something else you should think about. The smoke detector is designed to "chirp" when the battery is low. You will want to write your firmware to distinguish between a short duration "chirp" and a constant on "alarm."

As for the interface, I'm not sure that I agree with Andy that the output needs to drive both the piezo-sounder and the 8051 interface. I think it would be just as good to have the 8051 control the local report if one is called for. But he is (as usual) correct in the general sense. If people are or might be around, make certain you have a local report.

One final thing. You said that this is an ionizing type smoke detector. This means that there is a small alpha particle radioactive source encased in the unit. It is a very low level source, but you should take care not to expose it. You should wear a breathing filter sufficient to prevent you from inhaling anything that might be an alpha particle source. If you did accidently inhale something from it, your lung(s) would then be in for some long term radiation exposure. Exercise Extreme Caution!