as of jan 16 IBM says it isn’t possible.

Currently you cannot. But today, it was announced that it will be available in the very near future.

“In a note to investors on Thursday, American Technology Research analyst Shaw Wu cited “industry and developer sources” who say the Cupertino-based firm will use a portion of its media presentation next Thursday to announce improvements to the touch-screen handset’s ability to work with Microsoft’s Exchange server and IBM’s Lotus Notes software.”

Source: http://www.appleinsider.com/articles/08/02/28/iphone_to_gain_better_exchange_lotus_notes_support_at_sdk_event.html