Did some more testing. It appears [electric] jetpacks are having other issues in SMP as well.
While wearing an electric jetpack and 3 pieces of nano armor:
-Mode key does not toggle hover mode. I tried a few different mode keys. If the mode key is held at the same time spacebar is held, ascent is slow, similar to hover mode. Hover mode works correctly in SSP.
-Landing takes fall damage that appears to be related to the total vertical distance traveled. Taking off from the ground, going just a little ways up and then landing (using the jetpack to keep a low velocity) results in zero or very small fall damage. (~half a heart) If I take off from the ground (~y=66), fly to max height, and then land, I will take several hearts of damage, even if I slow my descent using the jetpack. Finally, if I take off from the ground, go to max height, and then descend to near ground (don't land) and ascend back to max height a few times, when I land I will die from fall damage no matter how gently I touch down. This behavior is not present in SSP.
-Jetpacks do not give the 1.5 armor units they do in 1.64. This behavior occurs in SSP as well, not sure if it is intended.