In a statement on its official blog, British retailer Handtec has put a hold on further Galaxy Nexus shipments pending investigation of the volume rocker issue that seems to be plaguing some customers. Handtec appears to confirm that it has to do with 2G reception specifically, saying that "the tests are showing when in 2G mode the volume is acting erratically" — so if you've already got a unit and you spend all your time in 3G coverage, you might be able to skate by this issue altogether.
To be clear, this isn't an official action on the part of either Samsung or Google — it's strictly Handtec's call to stop shipments at this point. Whether other retailers follow suit in the coming days remains to be seen.
Thanks, @ielisdave!