I don't think it will be a subscription as there's no way to really implement this the way the App Store is set up. However, they will be able to do in-app purchases, so TomTom could offer the option to buy updated maps within the navigation app without requiring you to go back to the App Store to buy new maps. That will be cool, though I hope it's not going to be annoying with pop-up reminders about buying new maps every time you open it.
I think the app with maps will cost around $100, which is how much I paid for TomTom's navigation app for my Palm PDA years ago. But there could be some interesting price wars with Navigon releasing their own navigation app, too.
I know it's quite annoying the whole "later this summer." I think all the companies know that anything related to thew new 3GS especially MMS and turn by turn are in such demand if they don't meet a specific date people are going to be really angry. I can't blame them for not wanting to piss off their user base. We know how impatient we all are