While you might assume that the relatively few people who use a Chromebook as their main computer are more likely to have an Android phone than an iPhone, it would still make sense for Chrome OS to be capable of tethering to an Apple device for the purpose of data sharing. That isn't possible at present, but it could be coming soon.
An entry in the Chromium Gerrit, spotted by Chrome Unboxed, titled 'Enable kernel support for iPhone tethering through USB' confirms that the Chrome OS team is working on adding the feature for an upcoming release. While you can currently use an iPhone or an iPad as a wireless hotspot if you want to use your mobile data on your Chromebook, this uses far more battery and is less reliable than a wired solution, as pointed out in the Gerrit notes.
As things stand, if you connect an iPhone or iPad to a Chrome OS machine, they will show up as a connected USB device but without ethernet capabilities, since they use different drivers. With a bit of work, however, the Chrome OS team will get the two systems talking to each other so we can expect to see USB tethering over iPhone in a future update. There's no telling exactly when it will come, but we'll keep an eye on developments.
- Chromium Gerrit
- Chrome Unboxed