I agree...if the iPhone wants to take away the market from all the Treo's and BlackBerry's..then it must have a MS Excel/Word/PPT viewer...iChat would be nice but I would much prefer Adium.
But the presence of select underpinings of OSX doesn't mean the interface will be easily accessible.
Given its connectivity, it should be relatively easy to poke around but it's not hard to imagine a scheme that would restrict available applications to those officially sanctioned by both Apple and Cingular.
Again, the Sidekick is a perfect example. Relatively great device. Goofy teeny-bopper interface. No 3rd party development (although there was some for awhile before Danger / TMobile put an end to it).
Since iPhone is a closed platform (at least initially), I think developers will offer iPhone-optimized websites as a workaround. Especially if iPhone ships with Flash client (which happens to be resolution independent to boot).