Sharepoint is very different from Public Folders.
There are actually two different types of Microsoft Sharepoint. Windows Sharepoint Services (WSS) and Microsoft Office Sharepoint Server (MOSS). Both are very similar with additional features being in the Server product. It is basically a collaboration web environment that uses IIS and a SQL Server backend. Sharepoint as a whole I consider a basic platform that can be extended to suit certain needs. You can create shared Calendars, Contacts, document folders, etc. You can setup different Sites and it supports I'd suggest taking a look at WSS, just to get a feel for it. I believe WSS itself is free.
http://technet.microsoft.com/en-us/windowsserver/sharepoint/default.aspx
If you use Outlook 2007, you can connect things from Sharepoint into your Outlook, by telling it to "Connect to Outlook". Depending on the type of object it is in sharepoint will be how it shows up in Outlook. Contacts, will show up in your Contacts. Calendars will show up under Calendars, etc. These will then end up showing under the Sharepoint Lists tab under Account Settings. What this means is that Outlook will keep an offline copy of the Sharepoint Data in Outlook.
As for ActiveSync, what it does is proxy sharepoint links through the Exchange 2007 Front-end servers into your network. Sharepoint has a simplified mobile interface. However not everything is necessarily viewable through that mobile view.
http://blogs.msdn.com/martinkearn/archive/2006/04/21/SharePoint-on-your-Phone_2100_.aspx
Not sure if the iPhone supports the proxying bit or not. This may not even be good enough to what you need as it is read only and not pushed to the device. You'd probably still be stuck using a third party product to get it to copy the data from public folders or sharepoint and put them into a folder within Outlook and be able to access it. I know iPhone supports multiple Contacts folders. Not sure if it can handle multiple Calendar folders (I know Windows Mobile doesn't).
I would not recommend Sharepoint in any form to be installed on the Exchange server. It can be very system resource intensive, especially if it starts getting used more.
You could get to Public Folders through IMAP (not supported in Exchange 2007), but I believe that still would not get what you need (a true calendar/contact view).
Sorry if I got your hopes up as this still doesn't look like it would provide you what you want. ActiveSync just isn't made for shared data synchronizaiton yet. Blackberry, Good, ActiveSync, etc all just provide a way to get to your intranet and not actually sync the shared data.
