There was a product called LipSync by a company called Kissworks. The company was, I think, just one person named Seth Ober. This is his current site: http://www.sethober.com/
His product was actually a Lotus Notes database that, somehow, synced all of Notes (calendar, mail, to-do, etc.) with iCal. From what I can see it was buggy and worked for some people and not others (check it out on Version Tracker.)
If you search the web, you can still find a download for LipSync--and Seth's website lets you generate a free license key for the product.
This would have been, I think the EXACT product you were looking for--me too. It just doesn't seem to work--or, if it does, Seth is no longer supporting it so there is no one to ask. For example, I am using Mac Notes 7; maybe it only works with an older version?
Maybe someone will be able to create a similar product...heck, maybe a bunch of us could chip in to make it worth Seth's while to update his handiwork? (I sent him an e-mail but he hasn't responded.)