Prof. Ed Felten, who attended the Alternative Compensation Systems conference this past week, has made a challenge to compulsory licensing proponents on Freedom to Tinker (Devil in the Details):

So here is my challenge to compulsory enthusiasts: tell us, in technical detail, how you propose to do the measurements. You don't have to give us working code, but do tell us which programs you would write or modify, and what specifically they would look for. Tell us how you would cope with backward compatibility, and the diverse formats in which people download and store music. Tell us how you would deal with non-PC platforms such as Macs, Linux boxes, and iPods, as well as non-traditional network setups such as public WiFi access points.

For example, monitoring can be optional for voting systems. If your devices support monitoring, then they can help you allocate how your "votes" are "spent" based on use. However, since it is a voting system, the monitoring can be optional.

For a Nielsen-type system the monitoring issue is somewhat easier since there are a smaller number of families/devices being monitored and they have volunteered for duty (meaning compliance would be higher). PC Software for the three major desktop systems (MS, Mac, Linux) that monitors P2P usage (downloads, uploads, plays) might be all that is necessary to be acceptable, given how imprecise Nielsens are in the first place. If necessary, there might be some extension of play monitoring to some mobile devices, perhaps specially configured devices provided free of charge to the Nielsen family.

The problems of monitoring for voting and Nielsen-type systems don't seem particularly challenging to me (unlike widespread monitoring of all users). However, voting and Nielsen-like systems create their own set of peculiar challenges.