Bombo! Phasing happens before the untap action, so they just untap right away anyway.

The intent is perfectly clear. If the rules don't work, that's the rules' problem.

(Saying that; how about "Permanents phase in tapped, and do not untap the turn they phase in")

I think it works as intended now. I wish it was a little more elegant.

Would it just be easier to write "Creatures that phase in cannot attack or block this turn"? Or is being tapped important?

By the by, I just noticed that this card is murder with... ooh, this thing just dodged Sands of Time. Any spell, however, that just phased lands every other turn would result in a strong lock...

I suppose not attacking is the same thing (barring Teferi's Isle)). They could still defend during the next player's turn, so being tapped is advantageous there. It doesn't make a huge difference.