Oh dear. Slide 39 in this Sony presentation - headed "Media/Investor Conference," and dated November 19 - appears to admit that some kind of subscription service is on the way to PSN.
PSN is listed as one of the key advantages of PlayStation 3, as it's to provide a "new revenue stream from subscription".
The slide also says that a "non-game software development kit" is to be released in the second quarter of 2010.
Rumour for now. We'll ask SCEE.