I've taken quite a few photos on the IPhone, post-processed them on a few apps and uploaded them to flickr via the Flickr for the iPhone and iPod touch app over the past couple of weeks. I noticed that somewhere or other the date/time stamp was getting lost. I've just done a bit of an investigation, taking a photo on the iPhone, uploading it via the iPhone app and comparing it against the same photo sync'd into iPhoto and exported
Via the iPhone app
Via the desktop uploader (this matches the EXIF via iPhoto and also Lightroom)
So, this tells me that I can't trust the date/time stamp on my images uploaded via the iPhone application to tell me when I really took the photo. Mildly annoying, but better to know for sure than to be guessing that is the case.