[Quick summary: delete reg value "HKLM\Software\Microsoft\Zune\ActivityTimer" to mitigate/cure music stuttering/skipping/freezing in Zune & Rhapsody]
Update: This still seems to be working as I'd hoped it would (on my device, at least - Samsung Focus w/ NoDo). With the value deleted, I can listen to an entire Rhapsody playlist in my car, streaming over 3G, with no skipping or freezing; re-creating the value brings back the skipping and the freezing.
The Zune Pass "stuttering" issue is rarer (for me, at least - other users have experienced other user experiences), but I haven't heard it in five days with the value deleted - re-creating it again just to verify that the original problem still repros.
I'm less-cautiously optimistic now about this "fix."
When I have time over the weekend, I'll post battery-usage comparisons. (I'm sure that ignoring user inactivity means higher power consumption for Zune - but just eyeballing the meter, it doesn't seem to be draining WAY faster.)