I've used StyleTap on my Windows Mobile devices and find it to be quite valuable in running Palm OS applications like Tide Tool (I still can't find a Windows Mobile version of an app like this). Robert just sent me a link to this video showing an experimental version of StyleTap running on an Apple iPhone/iPod Touch. It looks very slick and they even showed Tide Tool running at top speed. Depending on how the SDK and the 3rd party application system works (security, distribution, and business terms to be determined), we may see the same Palm OS applications for StyleTap on the iPhone/iPod Touch, Symbian, and Windows Mobile platforms very soon.
If the StyleTap developers can bring StyleTap to the iPhone/iPod Touch, that will open up a huge collection of 3rd party applications for end users and will make my iPhone so much more functional.
Development of the Symbian client was announced a couple of months ago and is getting close to a beta release as well.