One of the features of Keynote that I have always liked is the ability to save a presentation as a movie file. As once a movie file it can be converted in many different ways.
In version 4.0 of Keynote (part of iWork ’08) you can now send your presentation direct to YouTube.
This avoids the need to export the file and then upload to YouTube, you can upload direct to YouTube quickly and easily.
I’ve not yet tried it, but I can see after attending a conference I could upload my presentation and then embed it into my blog or the organisers could embed into their website. It also avoids the problems that you can have with Keynote files as not everyone has Keynote and even if you export as PowerPoint, not everyone has PowerPoint.