Obviously Apple wants to sell touch updates, and, in order for that to happen there has to some noticeable, tangible, perceived value that's enough of an incitement for end users to fork over the cash.
A bug fix to existing features isn't enough - Apple is very unlikely to charge for any such update since there is little incentive for most users to pay for it (and such a move would likely do more to piss off end users than anything else). If there are major performance improvements and/or additional useful features that have a marketable value, Apple will likely charge.
Right now, there isn't enough info about 2.2 to say what's in store.
A bug fix to existing features isn't enough - Apple is very unlikely to charge for any such update since there is little incentive for most users to pay for it (and such a move would likely do more to piss off end users than anything else). If there are major performance improvements and/or additional useful features that have a marketable value, Apple will likely charge.
Right now, there isn't enough info about 2.2 to say what's in store.