it's sort of normal. the 3800 makes enough torque at an idle to move the car forward.
there's a table in the PCM that sets idle RPM depending if you're in park/neutral or drive. When you shift into gear, it puts load on the engine. The PCM commands the idle air controller to open so the engine doesn't stall. Its weird the RPMs *jump* to 800. Typically the idle RPM in gear is a little lower than in park or neutral.