People have inexplicably made waves with commentary about "free to play" WoW due to a surprise change to the game's free trial program. There is no longer a time limit on the previously 14-day free trial, putting the trial on the same footing as similar programs in Warhammer and Conan (until that game's actual free to play relaunch anyway), but that change is less significant than it sounds.
Extended capabilities?
As far as I can tell, the only real changes are that trial players now have longer to reach level 20, and that all players (trial or otherwise) now get access to the TBC expansion (including Blood Elves, Draenei, and the two starting areas that did NOT get revised in Cataclysm). It sounds like "upgrading" to a real account permanently cuts off your "free to play" access to that account, so the real impact of these changes seems limited. Trial accounts are banned from joining guilds or inviting other players to groups, so there's no real way for a community of trial-capped 20's to form and push the limits of what can be accomplished at that level; most trials will eventually have to convert or quit.
That said, if you do not have an active WoW account, don't care about all the previously existing restrictions on trial accounts, are willing to put new alts on a throwaway trial account you may never use again to avoid sending Blizzard any money, and wanted to check out some of the new lowbie content in Cataclysm, this change may be for you. You could have done all of this before by signing up for new trial accounts every 14 days, but I can imagine some advantage of a more open-ended timeframe in which to do so.
If this is your plan, you may find my guide to Cataclysm starter zone revamps (no Goblins or Worgens for trial accounts) and old world leveling paths useful. By my count, Trial accounts have full access to sixteen zones, plus however much of the content in the four level 20-25 zones you are allowed to access at level 20, so that's a fair amount of content to visit.
Business impact
Free TBC, on the other hand, is potentially significant for retailers. Basically every physical or online store that carries computer games has boxed copies of TBC, and it appears that these should no longer be sold to anyone because it is no longer possible to own a plain vanilla WoW account. If this is the case, we might be talking about literally a million obsolete boxes at various stages of the retail supply chain, which likely puts this issue at an entirely different scale than previous cases of out-dated expansions or closed MMO's with boxes still left on store shelves.
That aside, it now costs $10-20 less to buy your way up to the current expansion, but my guess is that most people who wandered off before hitting level 20 and/or 2 weeks weren't coming back anyway. There are a few folks who might finally make it to level 20 one hour at a time, and a few folks who choose to stay perpetually at level 1-20 for free, but I don't see this move making a huge difference to the subscriber count.