Naaah. Ironically, I can tell you exactly what they are doing, having gone through a similar situation before. They cannot let go of their existing S60 UI because it is too deeply entrenched in their production process. To drop S60, they need time and resources to implement the new UI from scratch, test it, write documentation, prepare SDKs, etc., all the time supporting their current S60-based handsets. In fact, this is exactly what Nokia is trying to do by transitioning from S60 to QT, but is simply not possible to do in such a short time. They should have started much earlier, and they seem to have started when the saw the iPhone.

Not really, both use WebKit based browsers. The site actually opens without quirks on 5800 as well, but it is slow to load over GPRS and the site layout isn't quite made for a tiny touch based device. These limitations have nothing to do with underlying OS or the UI.