OK - here's more (just in case this thread was slowing down).
Here's the website of the Chief Counsel, you can look up official interpretations:
http://www.faa.gov/about/office_org.../agc/pol_adjudication/agc200/Interpretations/
I entered "tow" in the search field, and got the attached document.
It refers to 61.118. Apparently, 61.118 was later renumbered to 61.113.
The opinion was written in 1990 and is in response to questions from a glider club that was asking the same question.
I could not find the questions that resulted in this response though. The Chief Counsel, on the last page, suggests that PP tow pilots either not log the time, or get their commercial.
I don't know if changing 61.118 into 61.113 voids this interpretation, or if another ruling later overrules it.
edit:
After reading a little closer, the letter refers to 61.118 and says "Except as provided in paragraphs (
a) through (
d) of this section, a private pilot may not act as pilot in command of an aircraft that is carrying passengers or property for compensation of hire; nor may he, for compensation or hire, act as pilot in command of an aircraft."
The newer reg, 61.113 starts with "(a) Except as provided in paragraphs (
b) through (
g) of this section, no person who holds a private pilot certificate may act as pilot in command of an aircraft that is carrying passengers or property for compensation or hire; nor may that person, for compensation or hire, act as pilot in command of an aircraft."
Is it correct to say that since the reg has changed (not just from 118 to 113, but in content as well) that this ruling no longer applies?