The only way my dad says he'll get the iPhone is if TomTom's software will speak out street names. It doesnt do it on his current TomTom device, but it does on VZ Navigator on his Razr.
Way too early to tell. There is no guarantee that TomTom is going to release the software and even then only they would know if it will speak street names.
I remember reading an interview with the TomTom CEO where he said Apple's limitations is not a problem. I would copy and paste a link, but... I have yet to find the feature on my phone.