Over the history of Mac OS X, the installed base has adopted new OS releases at a very impressive rate. From the data available from Omni Group, this appears to have dramatically slowed with Leopard: in nearly a year since Leopard's introduction, it is still not used by the majority of systems reporting data to Omni.
Given the improvements in Snow Leopard and the desirability for Apple and third-party developers of having the user base on the latest OS, the impact on the ecosystem of having a free Snow Leopard release would be very nice.
The question that looms over all Apple upgrades these days concerns whether the company's interpretation of the Sarbanes-Oxley act will allow them to do it. In what follows, bear in mind that I am neither American nor an accountant, just an active observer.
As I see it, Apple's take on Sarbanes-Oxley is that the company cannot ship software updates which add meaningful features without either spreading the revenue for that product over a number of quarters (as with iPhone) or charging for the update (as with iPod touch and the 802.11n Enabler). There may be some debate over this interpretation, but it appears that this is how Apple sees it. In any case, given the recent history of the Apple board and the SEC, a period of conservative accounting is probably quite wise.
My understanding is that the only products Apple accounts for on a subscription basis are iPhone and Apple TV. As far as I know, revenue from Mac OS X sales is booked in the relevant quarter.
The key question, then, is whether Snow Leopard is an enhancement to Leopard or a distinct product. The WWDC mantra of "no new features except Exchange" for Snow Leopard muddies the waters slightly, but it seems clear to me that any OS release that carries a new marketing code name is a new product. Puma, Cheetah, Jaguar, Tiger and Leopard are all distinct products. 10.5.5 is fairly clearly a maintenance enhancement to 10.5.0.
Will it happen? I have no idea but, as an investment in helping prevent the installed base from fragmenting over three OS versions, I can see a good technical argument for doing it.