Has anybody though about a packet over sonet solution for an exchange?

As a matter of fact, yes. A SONET MPLS switch makes for a very interesting exchange. The use of MPLS avoids having the box act as a router and thereby avoids the headaches of figuring out just which routes the exchange should select. If the label switched paths in the switch are manually configured, there's no easy way for a non-neighbor to "accidentally" send you traffic. The switch fabric scales up nicely in that there is an interesting selection of link speeds that promise to scale up for the forseeable future. You can scale the fabric up either by growing individual switches, or by creating a switch fabric, or both. Sites connecting to the switch can do so remotely by bringing in a SONET link, so no on-site equipment is necessary. In short, technologically, a SONET MPLS exchange is a fine alternative because it provides the architectural advantages of a switched fabric without the bandwidth limitations of LAN technologies and without the cell loss problems of ATM. Note that this probably does not fix the economics and politics of the exchange point. Those are almost orthogonal issues. Tony