iPhone, Android and Windows Mobile
I have used the iPhone 3GS, Touch Diamond (WM6.1) and HTC Desire (Android 2.1) all with Exchange ActiveSync (from Exchange Server 2003) and Hotmail and here's what I've found.
iPhone - displays HTML beautifully from both Exchange and Hotmail and syncronises all Exchange folders with any changes on the phone being reflected elsewhere. However, it downloads Hotmail using POP3 so you can only see the Inbox and e-mails sent from the phone and any changes made on the phone are not reflected elsewhere. I believe there is an app that sorts out the Hotmail problem but I haven't tried that.
Windows Mobile - as stated in previous posts Exchange does not support HTML format in "over the air" syncronisations so all you get with Exchange is plain text if using Exchange Server 2003. Hotmail can display HTML and also allows you to sync all folders with any changes on the phone being reflected elsewhere.
HTC Mail on Desire- Handles Exchange Mail like Windows Mobile (plain text) and Hotmail like iPhone (via POP3 so only Inbox and no "proper" sync). In other words it seems to take the worst of both worlds.
What frustrates me is that the technology clearly exists to allow Exchange (using Exchnage Server 2003) to display HTML and to allow Hotmail to Sync properly. If all of these companies got together (some chance) and let the others licence their tecnology then the end user (me!) would be able to own whatever phone they wanted and have Exchange working like it does on the the iPhone and Hotmail working like it does on Windows Mobile.
Maybe one of these days!