I know I am bit late for the topic, but its worth to start on it now...
ActiveSync was always there for Microsoft Exchange, but now others are also adopting this approach for their push mail solutions.
Google (Gmail) has already started it and now activesync users of Android, iPhone, Symbian S60, Others can use activesync program of their mobile device and can sync their mails, contact, calendar with Gmail or with google Apps.
The new entry to this segment is from IBM Lotus Notes, with their Lotus traveler 8.5.1 version they have introduced Activesync support for iPhone and I guess sooner or later it will be for all other mobile devices as well.
As a solution architect or if I think from any company's perspective then I would like to have my own mobile client installed at mobile devices.
But from end-users perspective I would like to have a common client like activesync which can sync to Exchange, Google, Lotus Notes.
Well, bottom line could be until you have got something like blackberry control for end user mobile device you better off support the generic software.