We have a tempurpedic bed and absolutely love that thing! It is worth every penny and we get so much better quality sleep. We never coslept with ds but are planning on cosleeping with all other future children and I have often wondered the same thing. But to be honest I really don't think it would be bad. The tempurpedic is actually a much firmer matress than most other mattresses. And firm is the major qualification to make a matress safe. It depends what model you get (I believe there are 3) but the more expensive you get, the more it sinks down b/c there are more layers of the tempur material. We have the middle model which is what most people have, and I think it is still firmer than our old matress. I wouldn't get the highest model though (the "celebrity bed") because it probably wouldn't be firm enough. You do sink down but you will be suprised how little you actually sink. It feels like you are laying flat and not sinking at all, but that is just b/c it fits into the different parts of your body. If you have ever used a tempurpedic pillow or knock off, they sink down way more then the actual matress.
Another reason I think the tempurpedic would be safe for cosleeping is that it actually has air chambers in the middle which help the matress not to trap heat. Overheating has also been linked to SIDS so I think in that respect it makes it a safer matress. Further more you baby's weight will not be heavy enough to depress the matress even alittle bit, if at all.
One side note, it may take you a week or 2 to get used to the bed because it is so different but I think you will really like it.
