Before getting up on the stage, I still prefer to browse through an iPhone version of my talk (before I had the iPhone I used an iPAQ and it worked there, too). The iPhone can deal with native PowerPoint but seems to have difficulties with Keynote (I tried using Air Sharing and MobileFiles and it tells me the file type is unsupported – really? very odd).
So even if you get your presentation onto the iPhone using Air Sharing or whatever the iPhone may struggle if the presentation file is heavy on the graphics because of insufficient memory. Presentations used to be around 500-1000Kb in size a few years ago but we are all adding more graphics than before so it’s not unusual to see files of 10-20Mb.
Solution? I convert all my presentations to PDF files and then I use Adobe Acrobat (not the Reader) to reduce the file size. This will create a tiny file containing all your complex graphics. Perfect for viewing on the train, in the auditorium etc.