I think they'll be a long way off getting money from every iPod sold. For a start its such an illogical thing to ask for (Did the music companies ask for money for every CD player or Tape Recorder sold? Nope), plus I suspect the main reason that Microsoft agreed to pay money in the first place is that they needed to get the music labels on board to boost the Zune Music Store, Microsoft was in the weaker position here and I believe the labels exploited that weakness.
If the labels were to go to Apple and demand a royalty on every iPod and threatening to pull their catalogue if they didn't get it, they would actually come off worse than Apple in terms of lost revenue and it's because of this I reckon they haven't a chance...