So your last GPS-assisted trip included a road closure the navigator didn't take into account? Can't wait for the next map-update download to (hopefully) see if it's taken into consideration? Here's a better idea: pencil the change in yourself.
That's what you can do with the latest TomTom for iPhones app. Version 1.6 adds a "Map Share" component that lets you note major routing changes and (optionally) share them with TomTom; once the head Toms have verified your updates, they'll be added to a pool of other user-supported revisions that you can download and add yourself.
The newest release will set you back US$39.95 (ten bucks more north of the Canadian border), with an in-app option to purchase 12 months of TomTom Traffic service for US$19.95. Upgrading to iOS 4.x is strongly suggested; no word on whether (or if) there's a reduced upgrade rate if you recently purchased the previous version of the app.
[Purchase from iTunes]