Don't be confused by Apple's version numbering scheme for Mac OS X. The "ten" part was/is a big deal, so they're keeping that number around for a while. But 10.7 is a major version upgrade from 10.6, like upgrading from Windows Vista to 7.
You usually pay for these upgrades. There was an exception very early on, where 10.1 was free; but that was because 10.0 was the first buggy release. Also, the most recent upgrade was less expensive than usual, because there were fewer user-facing features, and they wanted to encourage people to upgrade to the newer internals. Apple of course can make another exception, if it aligns with their business goals. But there has been no announcement either way.