For a long time - in fact, from the beginning if my memory serves correctly - Skype has "promised" that Skype-to-Skype calls will "always be free". Well, it turns out that promise is worth exactly as much as any other promise Skype has ever made, or will ever make for that matter. A big, fat nothing.
Skype calls made on the iPhone via 3G will be "free" only until the end of this year. They originally tried to make the cutoff even earlier - August 2010 - but have apparently retreated to a later date as a result of a huge protest from Skype users. They don't mention it specifically, but you can bet your last dollar that the same "Whoops, no longer free" policy will apply to other mobile/3G versions.
Everyone here who is surprised raise your hand...