Eastern Arc is a new configuration that DISH is gearing up on. So far, it will consist of service offered from the 61.5 orbital slot. It should also include service from 72.7 in the future as DISH has negotiated broadcast rights from that location from its owners, TeleSat of Canada. Speculation includes the possibility of broadcasts coming from the 77 orbital slot, but that is not a real certainty.
Eastern Arc will be different in that it will be an all MPEG4 service possibly using an 8psk modulation scheme which means excellent efficiency in delivering channels, so a greater channel capacity will be realized. For many customers who currently need a dish at 110/119 and at 61.5 for HD, this will permit the ability to have a single dish pointed at the Southeastern sky instead of two separate dishes.
It is foreseeable that DISH may gradually transition customers over to using Eastern Arc or a future Western Arc (148 + others) that can be MPEG4 8PSK as well, leaving 110/119 to be for the MPEG2 systems until such time that a full conversion of old customers is financially reasonable, facilitating an all MPEG4 service from three different Arcs.